Different Multi-layer Photonic Configurations for Light Filtering in Optical Communications Rabiul Islam Sikder † , Muhammad Fasih † , Zunnoor Fayyaz Awan, Hyeonho Yoon, Hyo-Hoon Park, and Hamza Kurt School of Electrical Engineering Korea Advanced Institute of Science and Technology, Daejeon, South Korea rabiulsikdereee@kaist.ac.kr, muhammad.fasih@kaist.ac.kr, zunnoor.awan@kaist.ac.kr, exhyho@kaist.ac.kr, parkhh@kaist.ac.kr, hamzakurt@kaist.ac.kr Abstract— In this paper, we study the optical properties of periodic and quasiperiodic multi-layer configurations for light filtering in optical communications. The reflection spectrums of periodic and quasiperiodic multilayer structure are obtained using the finite-difference time-domain method. The optical filtering properties of the multi-layer structures are shown by comparing the reflection spectrum with the ITU-T dense wavelength division multiplexing standard. Keywords— multi-layer structure, optical filter, quasiperiodic structure I. INTRODUCTION Photonic crystals are a class of periodic nanostructure with periodic dielectric indices. Light interaction with the photonic crystal causes a few forbidden frequency intervals to appear in the transmission spectrum which are named as photonic bandgaps. The transmission spectrums of the photonic crystals are studied in microwaves, terahertz and visible frequency intervals [1-3]. Optical waves with frequency equal to the photonic bandgap cannot propagate in the forward direction and thus gets reflected in the backward direction. The reflected components can be treated as a stopband filter in optical communication. Recently, one-dimensional photonic crystals and their quasi-periodic version have shown excellent optical properties [4-10]. The multi-layer (1D) photonic crystal is considered as a common configuration in tuning the properties of optical devices. Therefore, they are considered as suitable alternatives for multichannel optical filters, low-loss waveguides, high reflectance mirrors, micro cavity and Omni guide fibers [11]. Among the various multi-layer photonic crystals, the periodic and quasiperiodic multi-layer configurations are considered because their transmission spectrums are dense in wavelengths. In this paper, we have studied the periodic and four quasi-periodic (Cantor, Fibonacci, Thu-Morse and Rudin- Shapiro) multi-layer photonic crystals for their potential application in optical dense wavelength division multiplexing (DWDM) filtering and multiplexing/de-multiplexing applications. The reflection spectrums were calculated using the FDTD method. We have studied the light filtering behaviour with free spectral range (FSR), number of narrow bands and bandwidth of each band in the reflection spectrums by taking into consideration of the ITU-T standard. Our work is organized as follows. In section II, an overview of different periodic and quasiperiodic multi-layer structures is presented. In section III, the reflection spectrums for different multilayer structures are provided. Finally, section IV concludes the article. II. OVERVIEW OF DIFFERENT MULTI-LAYER CONFIGURATIONS One-dimensional multi-layer structures consist of two different repeated alternating dielectric structures of layers A and B as shown in figure 1. The regular periodic structure has a period of Λ = dA + dB, where dA and dB are the thicknesses of layers A and B. To get maximum reflectivity of the central wavelength, quarter wavelength formula is used as dA = mλo/4nA and dB = mλo/4nB, where nA and nB are the refractive indices of layers A and B, respectively, and m is an odd integer value [12]. Figure 1. Regular 1D periodic dielectric structure Quasiperiodic multi-layer structure consists of two layers of A and B which are arranged sequences of Cantor, Thue-Morse, Rudin Shapiro and Fibonacci. In quasiperiodic structures the layers are neither periodic not random. A one-dimensional Cantor quasicrystals consist of alternating binary letters, A and B. The structure is formed with dielectric layers, A and B in the pattern A → ABA and B 293 International Conference on Advanced Communications Technology(ICACT) ISBN 979-11-88428-08-3 ICACT2022 February 13 ~ 16, 2022