New Design of Optical Add-Drop Filter Based on Triangular Lattice Photonic Crystal Ring Resonator

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1 International Research Journal of Applied and Basic Sciences 2013 Available online at ISSN X / Vol, 4 (4): Science Explorer Publications New Design of Optical Add-Drop Filter Based on Triangular Lattice Photonic Crystal Ring Resonator Zohreh Rashki 1*, Mohammad Ali Mansouri-Birjandi 2, Mohammad Reza Rakhshani 3 Faculty of Electrical and Computer Engineering, University of Sistan and Baluchestan, P. O. Box , Zahedan, Iran. Corresponding author zohreh_r81@yahoo.com ABSTRACT : In this paper, a new design of add-drop filter based on two-dimensional photonic crystal ring resonator is proposed. The structure is made of a triangular lattice of silicon rods with the refractive index n 1 = which are perforated in air with refractive index n 2 =1. Resonant modes of the all ring resonator and their corresponding degenerated poles are calculated using the Plane Wave Expansion (PWE) method and the filter s transmission spectrum is calculated using two-dimensional Finite Difference Time Domain (2D-FDTD) numerical method. Full Width Half Maximum (FWHM) bandwidth of the filter at the output transmission spectrum - from 1.507~1.511m - is 4nm.The dropping efficiency and quality factor of channel drop filter are 100% and , respectively. The proposed structure is small and the overall dimension is 10m 18.5m which is suitable for photonic integrated circuits. Keywords: Add-Drop Filter, Photonic Crystal, Ring Resonator, Triangular Lattice. Abbreviations: ADFs- Add-drop filters,2d-two-dimensional, FDTD-Finite Difference Time Domain, PWE-Plane Wave Expansion, FWHM-Full Width Half Maximum, PCs-Photonic Crystals, PBG-Photonic Band Gap WDM-Wavelength Division multiplexing, CDFs-Channel Drop Filters, PCRR-Photonic Crystal Ring Resonator INTRODUCTION Since 1987, the science of using photonic crystals is rapidly developing and receives special attention by the scientific and research communities (Robinson et al., 2012). Photonic Crystals (PCs) are composed of periodic dielectric or metallo-dielectric nanostructures that have alternate low and high dielectric constant materials (refractive index) in one, two or three dimension(s), which possesses Photonic Band Gap (PBG), where the transmission of light in certain frequency range is absolutely zero (Joannopoulos et al,2008; Fan et al.1998). Photonic crystals are very suitable candidates for realization of future passive and active optical devices because of their ability to control light-wave propagation, high speed of operation, better confinement, long life period and suitability for integration (Mahmud et al., 2012; David et al., 2012). By introducing some defects (point and/or line and/or both) in these structures, the periodicity and thus the completeness of the band gap are disturbed and the propagation of light can localized in the PBG region. This can lead to design of PC based optical devices in the PBG region (Inoue et al., 2004). In recent years, many of optical devices made were based on PCs such as multiplexers, de-multiplexers (Manzacca et al., 2007; Ghaffari et al.,2008), polarization beam splitters (Ghaffari et al.2008; Zabelin et al.,2007), switch (Wang et al.,2010), add-drop filters, channel-drop filters and so on (Shih et al.,2009; Moghaddam et al.,2010; Monifi et al.,2008; Djavid et al.,2008; Robinson et al.,2010; Djavid et al.,2010). Add-drop filters (ADFs) plays an important role in a wavelength division multiplexing (WDM) system, which enables to transmit data at multiple carrier wavelengths simultaneously throughout optical fibers for the substantial demand of the optical transmission bandwidth in optical communication networks (Wang et al., 2010).Various channel drop filters (CDFs) exist, such as fiber Bragg gratings, Fabry Perot filters, and arrayed waveguide gratings and ring resonator. Resonant CDFs, which involve waveguide-cavity interaction, are other attractive applicants for this intention (Fan et al.,1987; Yablonovitch,1987; John,1987).Generally, the ring resonator based on ADF provides efficient wavelength selection, scalability, narrow line width, flexible mode design and small channel spacing (Robinson et al.,2012). In this paper a two-dimensional add-drop filters by using photonic crystal ring resonator is

2 designed and investigated. Simulation of the designed filter is carried out by 2D-FDTD method. PWE is employed to calculate photonic band gap. The rest of the paper is arranged as follows: in the second part of the paper, we discuss the design procedures. We discuss the simulation results in Section 3and finally in the last section of the paper we express the conclusions. Structure design The design in this paper is based on two-dimensional triangular lattice of silicon rods (refractive index n si =3.4641) in an air background (n air =1). The number of rods in the x-z plane is To find the best rod s radii for which the PBG is maximum in the TE polarization, we draw the gap map in terms of filling ratio (r/a). The best filling ratio that the broadest PBG occurs for it, obtains for r/a= The normalized frequency range corresponding to this ratio is a/ In addition, the PBG s corresponding wavelength range is 1.19~1.95m. Fig. 1 shows the PC s perfect lattice band diagram which is also known as PBG and it is calculated for r/a=0.17 using PWE method with TM polarization mode. The X axis represents the line connecting points of first Brillion zone (the smallest periodical space in the lattice structure) and Z axis shows the normalized frequency a/2c = a/ where is the angular frequency, a is the lattice constant (distance between centers of two adjacent rods), c is the light speed in the vacuum and is the free space wavelength. Plane Wave Expansion method is most popular method that is used for theoretical analysis of photonic crystal structures. This method can express periodic structures as a superposition of a set of plane waves. In addition, it can obtain an accurate solution for the dispersion properties of a PC structure, but due to considering propagation modes, transmission spectra and field distribution cannot be extracted. Dispersion diagrams of our structures are calculated using this method (David et al., 2012). Figure 1. Photonic crystal perfect lattice frequency band structure for TM polarization mode. Photonic crystal ring resonator design Compared to point or line defects, ring resonators offer scalability in size, adaptability in structure design because of vast design parameters and flexibility in mode design due to their multi-mode nature. Some of these parameters are radii of scattering rods and the dielectric constant of the structure (Robinson et al., 2012; Mansouri- Birjandi et al., 2008). In general, a ring resonator is positioned between two optical waveguides provides an ideal basic structure for add-drop filter such that power in one waveguide is transferred into the other through the resonance of the ring, which is used to add or remove a channel from the multiplexed input/output signals. In this paper, we propose a new design of add- drop filter based on photonic crystal ring resonators. The performance of

3 the device is calculated by the two-dimensional Finite-Difference Time-Domain technique in photonic crystal triangular lattice. Mahmoud et al. (Mahmud et al.., 2012) studied channel drop filter based on the photonic crystal ring resonators. They obtained a Q factor 196 and dropping efficiency 100% at 1450nm.Recently Robinson et al. (Robinson et al., 2012) reported a single and dual PCRR in square lattice for filtering applications.they achieved dropping efficiency and Q factor 100% and , respectively. Fig. 2 shows the schematic layout of ADF, which consistss of bus and dropping waveguides and diamond resonator (coupling element). Also, it has four ports, among them ports A and B are the input and transmission output terminals whereas ports C and D are forward and backward dropping terminals, respectively. In this design, with reducing 11% refractive index of ring resonator rods, we can achieve dropping efficiency and Q factor, 100% and at 1508nm, respectively. Full width half-maximum bandwidth of the filter at the output transmission spectrum - from 1.507~1.511m - is 4nm. Figure 2. Schematic layout of ring resonator based ADF SIMULATION RESULTS A Gaussian optical pulse, covering the whole frequency range-of-interest, is launched at the input port A. Power monitors were placed at each of the other three ports (B,C,D) to collect the transmitted spectral power density after Fourier-transformation. The filter s transmission features are calculated using the FDTD numerical method with perfectly matched layers (PML) absorbing boundary conditions. Fig. 3 demonstrates the numerical simulation results for the resonant mode at 1.508m and non-resonant mode at 1.485m. At resonant wavelength m, the electric field of the waveguide is completely coupled into the ring and reached to one of its output ports. Fig. 4 displays the numerical simulation results for the wavelengths range 1.483~1.538m. The filter s dropping efficiency is 100% at the 1.508m.

4 Intl. Res. J. Appl. Basic. Sci. Vol., 4 (4), ( , 9, 2013 Figure3. 3. The filter s FDTD numerical simulation results at (a) m (at resonance) and (b) m Figure 4. (Color online) Transmission spectral response of the proposed ADF CONCLUSIONS In this paper, we proposed a new design of add-drop add drop filter based on photonic crystal ring resonators. Resonant modes of the all circular ring resonator and their corresponding degenerate poles calculated using the PWE method; and the filter s transmission spectrum wavelengths were calculated using 2D-FDTD 2D FDTD numerical method. Full Width Half Maximum bandwidth of the filter at the output transmission spectrum - from 1.507~1.511 m - is 4nm.The dropping efficiency and Q factor of channel drop filter are 100% 1 % and , respectively. Our proposed structure is small and the overall dimension is 10 m 18.5 m which is suitable for photonic integrated circuits. REFERENCES Djavid M, Abrishamian MS Multi-channel Multi channel drop filters using photonic crystal ring resonators. Optik123: Djavid M, Ghaffari A, Monifi F, Abrishamian MS Photonic crystal narrow band filters using biperiodic structures. J. Appl. Appl. Sci. 8:

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