Tuning of Photonic Crystal Ring Resonators for Application in Analog to Digital Converter Systems

Transcription

1 International Research Journal of Applied and Basic Sciences 2013 Available online at ISSN X / Vol, 4 (12): Science Explorer Publications Tuning of Photonic Crystal Ring Resonators for Application in Analog to Digital Converter Systems Mehdi Saffari, Mohammad Ali Mansouri-Birjandi *, and Mohammad Reza Rakhshani Faculty of Electrical and Computer Engineering, University of Sistan and Baluchestan, Zahedan, Iran * Corresponding author mansouri@ece.usb.ac.ir ABSTRACT: In this paper we have proposed a tunable two dimensional (2D) photonic crystal (PhC) ring resonator configuration for application in optical analog to digital converter (ADC) systems. Quality factor and dropping efficiency at the resonance of our designed ring resonator are 202 and 100%, respectively. The quantity of transmission efficiency and quality factor (Q) are suitable for ADC applications. In this device the quality factor is considerably improved with respect to other published reports. We study parameters which have an effect on resonant wavelength in this ring resonator, such as dielectric constant of coupling, inner and whole rods of the structure. The area of the proposed structure is about 103μm 2. Simulations are performed using 2D finite difference time domain (FDTD) method. Keywords: ADC; Dielectric constant; Photonic crystal; Wavelength; FDTD method INTRODUCTION In recent years, the possible of using optical technology to improve the performance of the analog to digital converter (ADC) has attracted researchers interests (Wu et al., 2008). Photonic crystals (PhCs) that exhibit photonic band gaps (PBGs) have received considerable interest over last two decades (Mansouri-Birjandi et al., 2008). Due to existence of PBG, PhCs have applications in different areas of optical engineering such as optical filters (Djavid et al., 2012), modulators (Li, 2008), polarization splitters (Shuo et al., 2012), and lasers (Fyath et al., 2012) which may ultimately make preparations for photonic integrated circuits (PICs). With the wonderful progresses in digital signal processing technology, high-speed ADC has been developed. Because they have the potential applications in future PICs and optical communication systems, the researchers have been focused in these areas. So far, several topologies have been proposed for photonic ADC (Valley, 2007; Miao et al., 2006). Ring resonators are very useful structure fir ADC designing. Ring resonators in a specific wavelength, which is the resonant wavelength, localize electromagnetic energy from an input waveguide into the ring and then transmit it to drop waveguide. Ring resonators have been used as the building blocks for the synthesis of high-order optical devices (Robinson and Nakkeeran, 2012; Chu et al., 1999). In this paper, a new type of PhC ring resonator has been designed for selecting desired wavelength in ADC systems. We used PhC ring resonator to achieve a new type of ring resonator with high normalized transmission (100%) and acceptable quality factor (over 202) in third communication window. Its performance is investigated

2 using the 2D FDTD method with the perfectly matched layer (PML) absorbing boundaries conditions at all boundaries. The distinctive features of this structure are high Q factor and its tunability. The resonant wavelength of our proposed structure is tunable and can be used as the basic element for other devices as well. Design Of Photonic Crystal Ring Resonator A triangular ring resonator obtained by removing a ring shape of columns from a trinagular lattice of dielectric rods in air background is displayed in Fig. 1(a). The dielectric rods (GaAs) have a dielectric constant (ε r ) of 11.56, and radius r=0.2a is located in air, where a is a lattice constant. To minimize the effect of counter propagating mode resulting from back-reflections at the sharp corners of the ring, we add two scatterer rod at half lattice constant as shown in Fig. 1(a). This additional rod at left corners acts as a right angled reflector reducing the back reflection at the corresponding corner (Saghirzadeh Darki and Granpayeh 2010; Dinesh Kumar et al., 2004). to increase the transmission power of port C we can shape the ring as pseudo-circular. As shown in Fig. 1(a), the inner rods of the ring placed at the corners are shifted towards the center of the ring up to quarter of lattice constant, allowing the ring to be shaped like a circle. By putting a waveguide beside the ring resonator, the waveguide at its resonant frequency can be coupled to the ring resonator to trap the electromagnetic energy propagating in the waveguide and localize it in the ring resonator. Three output ports of the structure are labeled as A, B and C, shown in Figure1(a). SIMULATIONS AND RESULTS In this structure, band gap opens for the normalized frequency 0.29<a/λ<0.42 for TM polarization (in which the magnetic field is in propagation plane and the electric field is perpendicular), where λ is the wavelength in free space. The characteristic of the transmitted power is obtained with finite difference time domain (FDTD) method. FDTD is a time domain simulation method for solving Maxwell s equations in arbitrary materials and geometrics (Taflove and Hagness, 2005). Berenger s perfectly matched layers (PML) are located around the whole structure as absorbing boundary condition (Berenger, 1994). The FDTD simulation result of this ring resonator that shows the normalized optical power transmissions is shown in Figure 1(b). As shown in Fig. 1(b), the wavelength λ=1554 nm of the input port is removed from the upper waveguide and transmitted to the port C. The transmitted power efficiency in this wavelength is about 100%. For the proposed structure, the value of Q is obtained 202. Q factor can be calculated with Q = λ/ λ, where λ and λ are central wavelength and full width at half power of output, respectively. We note that the amount of 202 is a high quality factor for ring resonator based PhC. Djavid (Djavid et al., 2012) and Robinson (Robinson and Nakkeeran, 2012) proposed a PhC ring resonators. Q factor in their structures are about 70 and 128, respectively. Continue on with this paper, the outcome of varying dielectric constant of rods on ring resonator resonance wavelength will be studied. Tuning The Photonic Crystal Ring Resonator One of the most significant aspects of any filter is its tunability. Here we investigate parameters which affect resonant frequency in photonic crystal ring resonators. First of all, we change the dielectric constant of the whole rods. Three different curves are displayed in Fig. 2(a) for ε r -0.4, ε r and ε r As seen in Fig. 2(a), the proposed structure, when simulated with the different dielectric constants of whole rods equal to ε r -0.4, ε r and ε r +0.4, can select wavelengths of 1544 nm, 1554 nm, and 1561 nm, respectively. 4243

3 As shown in Fig. 2(a), by raising the dielectric constant of whole rods, the resonant wavelength of the device is increased accordingly. In other words, a red shift occurs in resonant wavelength. We can create the different refractive indexes by using electro-optic (E-O) or thermo-optic (T-O) material. We utilize electro-optic materials which change their refractive indexes in response to external electric field; also we can use the T-O effect caused by two-photon absorption (TPA) in Si to control the resonator s refractive index through the heat generated by optically produced carriers (Djavid et al., 2008). With localized change in inner rods dielectric constant, the resonant wavelength can be tuned. Fig. 2(b) shows the normalized power transmissions of the structure with three different dielectric constants of inner rods, ε r -0.4, ε r and ε r In similar way, dielectric constant of coupling rods can be changed. Figs. 3 show the normalized transmissions of the structure with three different dielectric constants of coupling rods. As shown in Fig. 2(b), our proposed structure, when simulated with the different dielectric constants of inner rods equal to ε r -0.4, ε r and ε r +0.4, can select wavelengths of 1550 nm, 1554 nm, and nm, respectively. (a) 4244

4 (b) Figure 1. (a) A photonic crystal ring resonator and (b) Optical power transmission spectrum of this structure. (a) 4245

5 (b) Figure 2. Normalized transmission of our proposed ring resonator for different dielectric constants of (a) whole and (b) inner rods. Based on the results illustrated in Fig. 3, with different dielectric constants of coupling rods equal to ε r -0.4, ε r and ε r +0.4, we obtained wavelengths of nm, 1554 nm and 1528 nm in port C, respectively. High drop efficiency with high quality factor appears practicable, which makes this ring resonator one of the most promising designs for future photonic analog to digital application. Figure 3. Normalized transmission of the proposed structure for different dielectric constants coupling rods. 4246

6 CONCLUSIONS A tunable 2D photonic crystal ring resonators had been introduced and investigated through FDTD method for using in future photonic analog to digital converter systems. By using a single ring resonator, we obtained the output power transmission near to 100%. We investigated the effects of ring s parameters such as inner, coupling and whole rods dielectric constant on the resonance wavelength. For this structure, it was shown that the resonance wavelength has been tuned by varying this factor properly. We have revealed that there is flexibility in propose of the photonic crystal ring resonators. Such structure may offer promising applications for analog to digital converters based on PhCs and other nanophotonic systems. REFERENCES Berenger JP A perfectly matched layer for the absorption of electromagnetic waves. J. Computational Physics. 14: Chu ST, Pan W, Sato S, Kaneko T, Kokubun Y, Little BE An eight-channel add/drop filter using vertically coupled microring resonators over a cross grid. IEEE Photonics Technology Letters. 11: Dinesh Kumar V, Srinivas T, Selvarajan A Investigation of ring resonators in photonic crystal circuits. Photonics and Nanostructures Fundamentals and Applications. 2: Djavid M, Abrishamian MS Multi-channel drop filters using photonic crystal ring resonators. Optik. 123: Djavid M, Ghaffari A, Monifi F, Abrishamian MS T-shaped channel-drop filters using photonic crystal ring resonators. Physica E. 40: Fyath RS, Al-mfrji AA Investigation of chaos synchronization in photonic crystal lasers. Optics & Laser Technology. 44: Li J-Sh Novel optical modulator using silicon photonic crystals. Optics & Laser Technology. 40: Mansouri-Birjandi MA, Moravvej-Farshi MK, Rostami A Ultrafast low threshold all-optical switch implemented by arrays of ring resonators coupled to a Mach Zehnder interferometer arm: based on 2D photonic crystals. Appl. Optic. 47: Miao B, Chen C, Sharkway A, Shi S, Prather DW Two bit optical analog-to-digital coverter base on photonic crystal. Optics Express. 14(17): Robinson S, Nakkeeran R PCRR based add drop filter for ITU-T G CWDM systems. Optik - Int. J. Light Electron. Opt. doi: /j.ijleo Saghirzadeh Darki B, Granpayeh N Improving the performance of a photonic crystal ring-resonator based channel drop filter using particle swarm optimization method. Optics Communications. 283: Shuo L, Shu-Guang L, Ying D Analysis of the characteristics of the polarization splitter based on tellurite glass dual-core photonic crystal fiber. Optics & Laser Technology. 44: Taflove A, Hagness SC Computational Electrodynamics: The Finite-Difference Time-Domain Method, Artech House, Inc. Valley GC Photonic analog-to-digital converters. Opt. Express. 15: Wu Q, Zhang H, Yao M, Zhou W All-Optical Analog-to-Digital Conversion Using Inherent Multiwavelength Phase Shift in LiNbO 3 Phase Modulator. IEEE Photonics Technology Letters. 20(12):