Two Dimensional Photonic Crystal based Four Channel Demultiplexer for ITU.T.G 694.2 CWDM Systems K. Venkatachalam *, S. Robinson, S. Umamaheswari Department of Electronics and Communication Engineering Mount Zion College of Engineering and Technology, Pudukkottai, Tamil Nadu, India venkatachalamece@gmail.com (Received 13 th September, 2016; Revised 26 th September, 2016; Accepted 27 th September, 2016; Published: 27 th September, 2016) Abstract- In this paper, four channel demultiplexer is designed using two-dimensional photonic crystal for Coarse Wavelength Division Multiplexing applications. The proposed demultiplexer consists of bus wave guide, output wave guide and a cavity. The channel selection is done by having a cavity with unique refractive index. The two dimensional finite difference time domain and plane wave expansion methods are employed to estimate the normalized spectrum and band gap of the proposed demultiplexer. The footprint of the proposed demultiplexer is about 129.96µm 2 which is very small and hence it is suitable for Photonic Integrated circuits. Key Words: Photonic Band Gap, Finite Difference Time Domain, Plane Wave Expansion, Wavelength Division Multiplexing. 1. I'TRODUCTIO' Fiber optics communication is a popular one in the telecommunication network and it is a very useful technology to us for enabling the internet. Photonic crystals (PCs) have become the worldwide popular studies and focus on high efficiency transmission rate to maintain light propagation in PC structure. PCs [1,2] are periodic and dielectric structure and they affect the propagation of electromagnetic waves (EM) in the same way as the potential in a semiconductor crystal that affects the electron motion in allowed forbidden energy bands. The main property of the PCs is Photonic Band Gap (PBG) [3, 4] that means the certain region in the frequency band to restrict photons propagated through this structure. The photons that at are allowed to travel are known as modes, and group of allowed parts are band and disallowed parts of wavelength are called PBG. This gives rise to optical phenomena such as inhibition of spontaneous emission, high reflecting unidirectional mirrors and low loss wave guide. By introducing the point and line defect inside the PCs, they break the PBG region and allow EM waves. There are several components such as filters [5], isolator [6], multiplexer [7], demultiplexer [8-11] are realized for optical communication in the range of manometer as PC offers compactness, speed of operation and long lifetime. A PC based demultiplexer is realized using the defect wave guide [12], coupled cavity [13], ring resonator [14], super prism [15] & directional couplings [16] and etc., one of the most popular designs for demultiplexer using defect wave guide. Generally defect wave guide have received great attention in research site as it offers the simple structure, wide spectral range, narrow channel spacing and easy wavelength selection. In optical communication, the Wavelength Division Multiplexing (WDM) [17, 18] system is useful for better bandwidth utilization for multiplexer and demultiplexer technology. WDM uses multiple wavelengths are transmitted over a single fiber. There are two types of WDM system. They are Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM). CWDM has fewer than eight active wavelengths per fiber while DWDM has more than eight active wavelengths per fiber. We have considered CWDM systems because it has a wide range of frequencies without light amplification. The wavelength range of CWDM system is 1260nm 1610nm with 20nm channel spacing. The channels cantered at 1510nm, 1530nm, 1550nm and 1570nm are considered in this present work. In this paper four channels demultiplexer [19-21] is designed using two dimensional PCs is proposed by using point defect and varying the index difference. The Plane Wave Expansion (PWE) is one of the numerical methods to analysis the electromagnetic wave in frequency domain for manipulating the PBG in 2DPC. But it could not predict backward reflections. Hence, another method is finite difference method (FDTD) is selected to analyze electromagnetic field distribution through 2DPC and extracting backward reflections. It is an effective and simple computational method [22, 23]. The remaining portion of the paper is discussed as follows: In Section 2 and 3, the PBG of the structure by introducing the defects and also design of demultilexer is explained. The simulation results of proposed structure are discussed in Section 4 and Section 5 concludes the paper. 2. STRUCTURAL DESIG' The proposed four channel demultiplexer is designed using circular rods in square lattice. The circular rods offers better filling factor and square lattice based design have an easy fabrication which is considered in this present work. The total number of rods in the demultiplexer is 21 21=441 rods. The distance between two rods is kept as 540nm which is termed as lattice constant and denoted by a. The radius and refractive index of the rods are 0.115µm and 3.46 respectively. Page 37
The band diagram of the proposed demultiplexer is shown in Fig.1. The first Brillouin zone is shown in Fig.2, which is considered to estimate PBG before introducing defects. The first Brillouin zone is accurately predicted the propagation modes and PBG for a periodic structure. The band diagram of the proposed structure is obtained using Plane Wave Expansion (PWE) method. The band diagram has one TM PBG and three TE PBG whose normalized frequency and its wavelength range is listed in Table 1.The first TE PBG is ranging from 0.2718 a/λ to 0.3965 a/λ and its wavelengths are spanning from1361 nm to 1986 nm. The first TE PBG is considered in this present work as lies our desired region. 3. PHOTO'IC CRYSTAL BASED DEMULTIPLEXER The schematic representation of single channel filter (or) demultiplexer is represented in Fig3. It is composed of bus waveguide, cavity and drop waveguide. The bus and dropping waveguides are formed by line defects. The micro cavity is consisted of inner rods and outer rods where the inner rods are formed by varying the radius of the rod and altering its original position. The inner rods are built by varying the position of the sides whereas outer rods are constructed by varying the size and index difference. The input signal is applied at marked input port. The output signal is collected at marked output port through power monitor. The input waveguide is called bus waveguide whereas the output waveguide is known as dropping waveguide. At resonance, the wavelengths are coupled form the bus waveguide and exits through the dropping waveguide and collected at the output port. The input is referred as A and output port is referred as B. Fig. 1: Band diagram of the proposed device Fig. 3: Schematic structure of PC based structure for single channel The normalized transmission of single channel filter (or) demultiplexer is shown in Fig 4. The resonant wavelength, transmission efficiency and Q-factor of the device are 1510nm, 85% and 325, respectively. Fig. 2: Brillouin Zone of 21 21 PC structure Table 1: Types of PBGs Normalized Frequency and its Wavelength range Types of PBG Normalized Frequency Wavelength Range TE PBG 0.2718 a/λ to 0.3965 a/λ 1361 nm to 1986 nm 0.5468 a/λ to 0.5340 a/λ 984 nm to 1011 nm 0.71948 a/λ to 0.70669 a/λ 750 nm to 764 nm TM 0.77703 a/λ to 0.79302 a/λ 680 nm to 695 nm PBG When the defects either point defect or line defect or both are introduced in the periodic structure, the first TE PBG is entirely broken which in turn, allows the modes to propagate inside the structure. The aforementioned principle is employed for complete transfer of desired signal from bus waveguide to cavity and drop waveguide. Fig. 4: Optical transmission spectra of single channel at λ=1510 nm. The proposed four channel CWDM demultiplexer is shown in Fig. 5 where each output port is responsible for dropping its designated channel. The channel selection is done by having the microcavity with unique refractive indexes. The demutiplexer is proposed to drop four different channels that are cantered at 1510nm, 1530nm, 1550nm and 1570nm in ITU. T. G. 694.2 CWDM systems. The refractive index of the micro cavity is playing a significant role for channel selection. The refractive index values for 1510nm (λ 1 ) is 3.26, 1530nm Page 38
(λ 2 ) is 3.36, 1550nm (λ 3 ) is 3.46 and 1570nm (λ 4 ) is 3.56. In addition, the radius of the inner rods is 100nm whereas outer rods are 145nm. The crosstalk between the channels is listed in Table 3. It could be calculated through crossover the channel from one to another. The crosstalk values are varied from -10 db to -26 db. Fig. 5: Schematic representation of proposed four channel demultiplexer The input signal is applied into the input port and its output spectra are obtained by calculating the power received at each output ports. 4. SIMULATIO' RESULTS Fig. 6 depicts the normalized output transmission of proposed four channel demultiplexer. The transmission efficiency, Q-factor and passband width of proposed demultiplexer at 1510nm is 80%, 225 and 8respectively. From Fig. 6, it is investigated that the channel spacing among the channels are 20nm. The resonant wavelengths of the four channel demultiplexer are 1510nm, 1530nm, 1550nm and 1570nm. The 20nm channel spacing is obtained by having cavities with 0.1 value of refractive index difference. The performance parameters for other channels are listed in Table 2. From the Table 2, it is observed that the channel spacing is 20nm and the output efficiency and Q-factors are meeting the requirements of ITU. T. G. 694.2 CWDM standards. Hence, the proposed demultiplexer could be implemented in Photonic Integrated Circuits. In addition, it is also noticed that the resonant wavelength is shifted into the higher wavelength while increasing the refractive index of the cavity. There is around 20nm resonant wavelength shift is attained for every increasing 0.1 refractive index. Fig. 6: Normalized transmission spectra of proposed four channel demultiplexer The electric field distribution of proposed demultiplexer at 1510nm, 1530nm, 1550nm and 1570nm is shown in Fig. 8 (a), 8 (b), 8 (c) and 8 (d), respectively. At ON resonance, the input signal from the bus waveguide is coupled to respective micro cavity and to dropping waveguide. At OFF resonance, entire signal is reflected back to the source. Fig. 7: Effect of Resonant Wavelength vs Refractive Index of Elliptical Ring Rods Table 2: Resonant Wavelength,Channel Spacing,Transmission efficiency,quality factor and Pass band width of four channel CWDM demultiplexer Channels (λ) Refractive Index (n) Resonant Wavelength (λ o ) (nm) Channel Spacing (L) (nm) Passband Width(w) (nm) Transmission Efficiency (η) (%) Q Factor λ 1 3.26 1510 20 8 98 225 λ 2 3.36 1530 20 6 88 216 λ 3 3.46 1550 20 10 100 154 λ 4 3.56 1570 20 6 99 296 Crosstalk of four channels(db) Table 3: Crosstalk of Four channel CWDM demultiplexer λ 1 λ 2 λ 3 λ 4 λ 1 - -14.31-26.072-16.07 λ 2-13.92 - -10.135-22.78 λ 3-19.52-12.31 - -19.30 λ 4-18.73-12.29-19.19 - Page 39
Fig. 8: Field distribution of proposed four channels demultiplexer at (a) 1510 nm (b)1530nm (c)1550 nm and (d)1570 nm 5. CO'CLUSIO' In this paper two dimensional PCs based four channel demultiplexer is proposed and designed for ITU. T. G 694.2 CWDM systems. The output efficiency, Q factor and resonant wavelength of each channel are evaluated. The channel spacing is 20nm. The average output efficiency is varied from 50% to 85% and the average Q factor is 223. The crosstalk of the proposed demultiplexer is low and the size of proposed demultiplexer is 129.96µm2 which is very much small for photonic integrated circuits and future optical networks. REFERE'CES [1] J. D. Joannopoulos, R. D. Meade, and J. N. Winn, Photonic Crystals: Molding the flow of light, Princeton University Press, Princeton, NI, USA. 1995. [2] A. E. Akosman, M. Mutli, H. Kurt, and E. Ozbay, Dual-Frequency division demultiplexer based on cascade photonic crystal waveguides, Physica B, vol. 407, no. 20, pp. 4043-4047, 2012. [3] A. E. Akosman, M. Muthu, H. Kurt, and E.Ozbay, Compact wavelength demultiplexer design using slow light regime of photonic crystal waveguides, Opt. Express, vol. 19, no. 24, pp. 24129-24138, 2011. [4] A. Kumar, B. Suthar, V. Kumar, KH. S. Singh and A. Bhrgava, Tunable wavelength demultiplexer for DWDM applications using 1-D Photonic Crystal, Prog. in Electromag. Res. Letters, vol.33, pp.27-35, 2012. [5] M. Y. Mahmoud, G. Bassou and A. Taalbi, A new optical add-drop filter based on two-dimensional photonic crystal ring resonator, Optik, vol. 124, no. 17, pp. 2864-2867, 2013. [6] K. Fang, Z. Yu, V. Liu, and S. Fan, Ultracompact nonreciprocal optical isolator based on guided resonance in a magneto-optical photonic crystal slab, Optics Letters, vol. 36, no. 21, pp. 4254-4256, 2011. [7] M. Koshiba, Wavelength division multiplexing and demultiplexing with photonic crystal waveguide couplers, IEEE/OSA J. of Light Wave Tech., vol. 19, no. 12, pp. 1970-1975, 2001. [8] M. Notomi, A. Shinya, K. Yamada, J. I. Takahashi, Structural tuning of guiding modes of line-defect waveguides of silicon-on-insulator photonic crystal slabs, IEEE J. of Quantum Elect., vol. 38, no. 7, pp. 736-742, 2002. [9] W.-H. Chang et al., Efficient single-photon sources based on low-density quantum dots in photonic crystal nano-cavities, Phy. Rev. Lett., vol. 96, no. 11, pp. 117401, 2006. [10] W.-Y. Chiu et al., A photonic crystal ring resonator formed by SOI nano-rods, Optics Exp., vol. 15, no. 23, pp. 15500-15506, 2007. Page 40
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