A narrowband tunable transmission filter suitable for wavelength division multiplexing is designed. The basic structure is a one-dimensional Fabry–Perot structure formed by layers of dielectric magnesium fluoride and electro-optic lithium niobate, which act as low and high refractive index material layers, respectively. A narrowband phase shifted transmission peak occurs within the stopband of the reflectance spectra of the structure by introducing the defect of a low-index material at a suitable position in the structure. The bandwidth of the peak depends on the number of bilayers and also on the operating wavelength. The phase shift of the transmission peak is linearly related to the wavelength under consideration. By adjusting the defect layer width, this shift of the transmission peak from the operating wavelength can be avoided. The device dimensions are so chosen that such a structure can be fabricated and used with presently available technology. A linear transmission peak tunability of 4 nm/10 V is achieved for this device by varying the refractive index of the electro-optic lithium niobate layer with externally applied voltage along its z axis. All the simulations have been carried out using the finite difference time domain method in a MATLAB® environment.
The translational symmetry of the periodicity in a photonic crystal can be disturbed by introducing a controlled defect in
its periodicity. The photon localization causes a pass band in the photonic bandgap. Based on this concept, we are proposing
the design of a tunable narrow band filter for multiple wavelengths used for coarse wave length division multiplexing
(CWDM) system. To achieve that, a multiple stack Fabry Perot structure with suitable stack materials and controllable
defect is considered. The proposed Fabry Perot structure consists of periodic layers of electro optic material Lithium
Niobate (nH) and Magnesium Fluoride (nL). The optical length of each such layer is their corresponding quarter wave
length width at the design wavelength λ<sub>0</sub> (1.55 μm). The reflection band of the quarter wavelength multilayer structure is
formed due to the periodic repetition of the (LH)<sup>N</sup>L, where L and H are the quarter wavelength width of the nL and nH
material respectively and N is the number of bilayers. A quarter wave layer L is then inserted between the groups of (LH)<sup>N</sup>
and (HL)<sup>N</sup> to form the Fabry Perot resonator structure which can be used for narrow band transmission filter. The
simulation has been carried out in both FDTD and TMM method and compared. As the refractive index of the Lithium
Niobate can be tuned electro-optically, the filter can also be made tunable.