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Channel drop filter for CWDM systems
Optics Communications 306 (2013) 179–184
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Channel drop filter for CWDM systems Mahmoud Youcef Mahmoud a,b,n, Ghaouti Bassou a,c, Frédérique de Fornel c, Ahmed Taalbi a a b c
Laboratory of Microanalysis, Microscopy and Molecular Spectroscopy, Faculty of Science, Djillali Liabes, University of Sidi Bel Abbes, Algeria Institut des Sciences et Techniques, Département des Sciences de la matière, Centre Universitaire de Relizane, Algérie Groupe d'Optique de Champ Proche, Laboratoire Interdisciplinaire Carnot de Bourgogne, CNRS, 9 Avenue A. Savary, 21078 Dijon, France
art ic l e i nf o
a b s t r a c t
Article history: Received 21 February 2013 Received in revised form 11 April 2013 Accepted 12 May 2013 Available online 14 June 2013
In this paper, a new design of channel drop filter (CDF) based on two-dimensional photonic crystal ring resonators (PCRRs) is provided by two-dimensional (2D) finite-difference time-domain (FDTD) simulations in triangular lattice photonic crystal (PC) silicon rods. 100% forward dropping efficiency and a quality factor of over 1000 can be achieved at maximum transfer efficiency while the operating wavelength is 1550 nm. Through this novel component, three channel drop operation with 100% dropping efficiencies at all output channels can be obtained. The proposed filter provides a possibility of channel drop filter and could be used in coarse wavelength division multiplexing (CWDM) systems. & 2013 Elsevier B.V. All rights reserved.
Keywords: Channel drop filter Two-dimensional photonic crystals Ring resonators FDTD CWDM systems
1. Introduction Photonic crystals (PCs) which can control light wavepropagation have attracted great research interest due to their potential applications in integrated optics, laser and optical communications. Many optical components have been designed and fabricated based on PCs such as waveguides [1–3], resonant cavity with ultra-high Q [4,5], optical switch [6,7] and channel drop filters [8–11]. The channel drop filters (CDFs) based on square lattice twodimensional photonic crystals (2D PCs) [12], or on triangular lattice hole structure [9], attracted extensive attention from those of conventional optical filters [13,14]. Fan et al. [12] proposed a channel drop filter composed of two identical resonant cavities sandwiched by two parallel waveguides which supports two resonant modes with opposite symmetries. They showed a complete channel drop tunneling processes. Noda et al. [15] raised another method to trap and drop photons into the vertical direction of the PC slab through the coupling of a single defect and a waveguide. They also proposed a method of tuning the resonant wavelength by changing the lattice constant of the PC structure in order to further construct a dense WDM (DWDM) system [15]. Zhang and Qiu [16] designed a compact in-plane
n Corresponding author at: Laboratory of Microanalysis, Microscopy and Molecular Spectroscopy, Faculty of Science, Djillali Liabes, University of Sidi Bel Abbes, Algeria. Tel.: +213 6 62 67 29 63. E-mail address: [email protected] (M. Youcef Mahmoud).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.05.032
channel drop filter using a single cavity that supports two high Q modes. By modifying the upper boundary of the drop waveguide, they achieved a high forward dropping. In principle, nearly 100% drop efficiency can be realized and then light can be collected in the normal direction of the PC slab. However, it is a real challenge to integrate this channel drop filter into a planar optical integrated circuit [17]. Thus, significant progress has been made on CDF based devices in the areas of compactness, high spectral selectivity, wide spectral tunability, fast switching, and low-power switching [18,19]. Photonic crystal ring resonators (PCRRs) are also used in the design of CDFs [20]. It can offer scalability in size and flexibility in mode design. Recently, several types of CDF based on 2D PCRR have been proposed using quasi-square PCRR [20], square PCRR [20], dual square PCRR [20], dual curved PCRR [21], hexagonal PCRR [22], 451 PCRR [23], circular PCRR [24], and X-shaped PCRR [25]. In the present paper, we propose a new design of CDF based on 2D PCRRs. The performance of the filter is investigated by using the two-dimensional (2D) finite-difference time-domain (FDTD) technique in triangular lattice photonic crystal (PC) silicon rods. The proposed device has appropriate performance and is very suitable for CWDM multi-channel systems.
2. Design and numerical results In this paper, the structure consists of a two-dimensional (2D) triangular lattice PC composed of silicon (Si) rods embedded in air
180
M.Y. Mahmoud et al. / Optics Communications 306 (2013) 179–184
Fig. 1. (a) W1 line defect PC waveguide, (b) 2D super-cell with a defect (a is the lattice constant), (c) Dispersion curves for TM polarization and the corresponding waveguide mode shown as a pink line in the photonic bandgap region. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
substrate. The refractive index of the rods is 3.46 and the radius of the rods is 0.2a, where a is the lattice constant. As shown in Fig. 1 (a), the W1 line defect waveguide in the PCs is realized by removing a row of dielectric rods along the ΓK direction in a triangular lattice PC that have a fundamental band gap for TM mode (Electric field is perpendicular to x–y plane). Fig. 1(c) shows the dispersion curves of the W1 line defect PC waveguide for the TM polarization through the plane wave expansion (PWE) [26] method using the 2D super-cell approach highlighted in Fig. 1(b).
The waveguide supports a single-mode frequency (normalized) ranging from 0.337 a/λ to 0.442 a/λ below the light-line, where λ is the wavelength of light in free space. To be used in optical communication systems (1550 nm), the lattice constant, a, i.e. the distance between the two adjacent rods, is set as 635.3 nm. Thus the W1 PC waveguide is broadband, with guided singlemode spans from 1437 to 1885 nm. The newly designed channel drop filter based on twodimensional (2D) triangular lattice PC consists of two waveguides
M.Y. Mahmoud et al. / Optics Communications 306 (2013) 179–184
181
Fig. 3. Normalized transmission spectra of the CDF at ports B, C and D respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Schematic diagram of the proposed CDF based on 2D PCRR.
and a single X-shaped PCRR sandwiched between them. As shown in Fig. 2, the top waveguide is called as bus waveguide whereas the bottom waveguide which is formed by removing a row of dielectric rods in ΓK direction but rotated 601 and 1201 from the horizontal is known as drop waveguide. The input, the forward transmission, the forward dropping and the backward dropping ports are labeled as A, B, C and D respectively. To create resonant states of different symmetry [12] and ensure complete power transfer from the input to drop port through the ring resonator, four gray rods (small rods) are reduced to 0.77r and shifted in the ΓK direction of 0.18a. The white rods have exactly the same diameters as all other dielectric rods in PC structure and they are also shifted of 0.248a in the opposite side of the ΓK direction. The transmission characteristics of the proposed CDF are calculated using the two-dimensional (2D) finite-difference timedomain (FDTD) technique [27], with perfectly matched layers (PMLs) absorbing boundary conditions. A TM Gaussian optical source is launched at the input port A and excites both the even and odd states, each with a Lorentzian line shape. Power monitors were placed at each of the other three ports (B, C, D) to collect the transmitted spectral power density after Fouriertransformation. All of the transmitted spectral power densities were normalized to the incident light spectral power density from input port A. The normalized transmission spectra at ports B, C and D are displayed in Fig. 3 as black, red and blue lines respectively. As shown, each of the transmission and the backward dropping efficiencies is absent at the resonant frequency, whereas high forward dropping efficiency is observed. On the other hand, all the power in the bus waveguide is extracted by using resonant tunneling process and well transferred into port C through the resonant ring. In this case, the proposed filter can be regarded as two coupled PCRRs with both forward and backward dropping capabilities depending on the mode parities. As shown from the movie in Fig. 4, the coupled mode in the bottom PCRR ring cavity
rotates in the clockwise direction with the propagating waveguide mode, whereas, the propagating wave in the top PCRR ring cavity propagates counter-clockwise which leads to the forward dropping. Note that the system can perform backward dropping if the modes propagate clockwise in both PCRRs. It is interesting that the forward dropping at the port C predominates, which is obviously different with the dominance of backward dropping at resonance for the traditional resonant single micro-ring structure. 100% drop efficiency and a quality factor larger than 1000 can be achieved while the operating wavelength is 1550 nm. The off-resonance transmission is close to 100% over the entire spectrum, except at the resonant frequency, where it drops to 0%. By comparing our new CDF with the related works in the literature, the spectral selectivity and the quality factor of the proposed one is higher, as shown in Table 1. The electric field patterns of the even mode (symmetric with respect to the plane perpendicular to the W1 line defect waveguide) and the odd mode (asymmetric with respect to the plane perpendicular to the W1 line defect waveguide) are obtained by launching a continuous-wave (CW) source into input port A at the resonant wavelength (λ ¼1550 nm) as shown in Fig. 4(a) and (b) respectively.
3. Demultiplexer design for CWDM systems According to the results shown above, we can design a multiCDF to simultaneously add and drop various wavelength channels from a single multi wavelength source using the resonant tunneling process. To illustrate this concept, three parallel cascaded ring resonators are horizontally coupled to a W1 line defect waveguide. The device consists of one bus waveguide and three drop waveguides with various refractive indexes. The variation of the refractive index has been selected to obtain a shift of the resonant frequency. As shown in Fig. 5, the diameter of small and whole dielectric rods in the PC structure remains constant, whereas the refractive index of region 1, 2 and 3 are 3.61, 3.46 and 3.31 respectively. The incident port and exit ports are labeled
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Fig. 4. Electric field patterns of the CDF at the resonant wavelength (λ¼ 1550 nm) (a) even mode and (b) odd mode (movie 599 Ko).
Table 1 Comparaison to other devices in the literature. Reference
Drop efficiency (%)
Quality factor
PCRR type
Our work Robinson and Nakkeeran [28] Ma and Ogusu [29] Bai et al. [23] Hsiao and Lee [30] Andalib and Granpayeh [21] Djavid et al. [31] Qiang et al. [20]
100 100 95 90 55 68 99 498
41000 114.6 775 840 423 153.6 52.7 160–1000
X-shaped Circular Diamand 451 Hexagonal Dual curved Quasi-square Quasi-square
Fig. 5. Schematic diagram of the multi-CDF.
as A, B, C, D and E respectively. Broadband light is inserted into the bus and resonant wavelengths specific to each ring resonator are individually demultiplexed into separate drop waveguides. By
reversing the input and output roles, the proposed design can also act as a multiplexer to combine various optical wavelength signals into a single bus waveguide.
M.Y. Mahmoud et al. / Optics Communications 306 (2013) 179–184
183
Fig. 6. Normalized transmission spectra of the multi-CDF at ports B, C, D and E respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The transmission spectra for the proposed structure at ports B, C, D and E are displayed in Fig. 6 as black, red, blue and green lines respectively. We confirmed by numerical calculation that an increase in index causes resonant wavelengths to shift toward longer wavelengths and a decrease in index causes a shift toward shorter wavelengths. On the other words, by increasing (decreasing) the refractive index of the rods, a red shift (blue shift) in resonant wavelength occurs. Resonance properties can be precisely tuned to select specific wavelengths by modulating the lattice constant or by further tuning such as the radius of the rods, the radius of small rods, and/or the refractive index of small rods. The forward dropping efficiencies are 100% at all output channels and the selected wavelengths are 1529.95 nm, 1549.79 nm and 1569.94 nm respectively with spacing of 20 nm corresponding to the ITU-T G.694.2 grid for CWDM systems. The electric field patterns at λ ¼1510 nm, λ ¼1529.95 nm, λ ¼1549.79 nm and λ ¼1569.94 nm are shown in Fig. 7(a), (b), (c) and (d) respectively.
4. Conclusion In this paper, a new channel drop filter based on photonic crystal ring resonators in two-dimensional triangular lattice silicon rods was proposed. 100% forward dropping efficiency and a quality factor of over 1000 can be achieved at maximum transfer efficiency while the operating wavelength is 1550 nm. Based on the proposed component, a multi channel drop filter with 100% drop efficiencies at all output ports can be obtained. The proposed filter provides a possibility of channel drop filter and could be used in coarse wavelength division multiplexing (CWDM) systems.
Acknowledgment The present work was supported by the Ministry of Higher Education and Scientific Research of Algeria.
Fig. 7. Electric field patterns at (a) λ¼ 1510 nm, (b) λ¼ 1529.95 nm, (c) λ¼ 1549.79 nm and (d) λ¼ 1569.94 nm.
References [1] A. Mekis, J.C Chen, I. Kurland, S. Fan, P.R. Villeneuve, J.D. Joannopoulos, Physical Review Letters 77 (1996) 3787. [2] S.G. Johnson, P.R. Villeneuve, S. Fan, J.D. Joannopoulos, Physical Review B 62 (2000) 8212. [3] A. Chutinan, S. Noda, Physical Review B 62 (2000) 4488. [4] B.S. Song, S. Noda, T. Asano, Y. Akahane, Nature Materials 4 (2005) 207. [5] M. Notomi, H. Taniyama, Optics Express 16 (2008) 18657. [6] M.F. Yanik, S. Fan, M. Soljacic, Applied Physics Letters 83 (2003) 2739. [7] M. Notomi, A. Shinya, S. Mitsugi, G. Kira, E. Kuramochi, T. Tanabe, Optics Express 13 (2005) 2678. [8] Y. Akahane, T. Asano, B.S. Song, S. Noda, Applied Physics Letters 83 (2003) 1512. [9] M. Imada, S. Noda, A. Chutinan, M. Mochizuki, T. Tanaka, Journal of Lightwave Technology 20 (2002) 873. [10] H. Ren, C. Jiang, W. Hu, M. Gao, Y. Qu, F. Wang, Journal of the Optical Society of America 24 (2007) 7.
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[11] M. Qiu, B. Jaskorzynska, Applied Physics Letters 83 (2003) 1074. [12] S. Fan, P.R. Villeneuve, J.D. Joannopoulos, H.A. Haus, Optics Express 3 (1998) 4. [13] I. Baumann, J. Seifert, W. Nowak, M. Sauer, IEEE Photonics Technology Letters 8 (1996) 1331. [14] M.Y. Park, W. Yoon, S. Han, G.H. Song, Electronics Letters 38 (2002) 1532. [15] S. Noda, A. Chutinan, M. Imada, Nature 407 (2000) 608. [16] Z. Zhang, M. Qiu, Optics Express 13 (2005) 2596. [17] Z. Yi-Nan, L. Ke-Zheng, W. Xue-Hua, J. Chong-Jun, Chinese Physics B 20 (2011) 074210. [18] M. Lipson, IEEE Journal of Lightwave Technology 23 (2005) 4222. [19] M. Notomi, A. Shinya, S. Mitsugi, E. Kuramochi, H.Y. Ryu, Optics Express 12 (2004) 1551. [20] Z. Qiang, W. Zhou, R.A. Soref, Optics Express 15 (2007) 1823. [21] P. Andalib, N. Granpayeh, Optical add/drop filter based on dual curved photonic crystal resonator, in: Proceedings of the 5th IEEE International Conference on Photonics, 2008, pp. 249–250. [22] T.T. Mai, F.L. Hsiao, C. Lee, W. Xiang, C.C. Chen, W.K. Choi, Sensors and Actuators A Physical 165 (2011) 16.
[23] J.B. Bai, J.Q. Wang, X.Y. Chen, J.Z. Jiang, L Hui, Y.S Qiu, Z.X Qiang, Optoelectronics Letters 6 (2010) 0203. [24] S. Robinson, R. Nakkeeran, Optoelectronics Letters 7 (2011) 0164. [25] M. Youcef Mahmoud, G. Bassou, A. Taalbi, Z.M. Chekroun, Optics Communications 285 (2012) 368. [26] S.G. Johnson, J.D. Joannopoulos, Optics Express 8 (2001) 173. [27] A.F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J.D. Joannopoulos, S.G. Johnson, Journal of Computer Physics Communications 181 (2010) 687. [28] S. Robinson, R. Nakkeeran, Optik-International Journal for Light and Electron Optics (2012). [29] Zatao MaKazuhiko Ogusu, Optics Communications 284 (2011) 1192. [30] Fu-Li Hsiao, Chengkuo Lee, A nano ring resonator based on 2D hexagonal lattice photonic crystals, in: Proceedings of the IEEE Conference on Optical MEMS and Nanophotonics ,2009 pp. 107–108. [31] M. David, A. Ghaffari, F. Monifi, M.S. Abrishamian, Physica E (2008)3151.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Channel drop filter for CWDM systems Mahmoud Youcef Mahmoud a,b,n, Ghaouti Bassou a,c, Frédérique de Fornel c, Ahmed Taalbi a a b c
Laboratory of Microanalysis, Microscopy and Molecular Spectroscopy, Faculty of Science, Djillali Liabes, University of Sidi Bel Abbes, Algeria Institut des Sciences et Techniques, Département des Sciences de la matière, Centre Universitaire de Relizane, Algérie Groupe d'Optique de Champ Proche, Laboratoire Interdisciplinaire Carnot de Bourgogne, CNRS, 9 Avenue A. Savary, 21078 Dijon, France
art ic l e i nf o
a b s t r a c t
Article history: Received 21 February 2013 Received in revised form 11 April 2013 Accepted 12 May 2013 Available online 14 June 2013
In this paper, a new design of channel drop filter (CDF) based on two-dimensional photonic crystal ring resonators (PCRRs) is provided by two-dimensional (2D) finite-difference time-domain (FDTD) simulations in triangular lattice photonic crystal (PC) silicon rods. 100% forward dropping efficiency and a quality factor of over 1000 can be achieved at maximum transfer efficiency while the operating wavelength is 1550 nm. Through this novel component, three channel drop operation with 100% dropping efficiencies at all output channels can be obtained. The proposed filter provides a possibility of channel drop filter and could be used in coarse wavelength division multiplexing (CWDM) systems. & 2013 Elsevier B.V. All rights reserved.
Keywords: Channel drop filter Two-dimensional photonic crystals Ring resonators FDTD CWDM systems
1. Introduction Photonic crystals (PCs) which can control light wavepropagation have attracted great research interest due to their potential applications in integrated optics, laser and optical communications. Many optical components have been designed and fabricated based on PCs such as waveguides [1–3], resonant cavity with ultra-high Q [4,5], optical switch [6,7] and channel drop filters [8–11]. The channel drop filters (CDFs) based on square lattice twodimensional photonic crystals (2D PCs) [12], or on triangular lattice hole structure [9], attracted extensive attention from those of conventional optical filters [13,14]. Fan et al. [12] proposed a channel drop filter composed of two identical resonant cavities sandwiched by two parallel waveguides which supports two resonant modes with opposite symmetries. They showed a complete channel drop tunneling processes. Noda et al. [15] raised another method to trap and drop photons into the vertical direction of the PC slab through the coupling of a single defect and a waveguide. They also proposed a method of tuning the resonant wavelength by changing the lattice constant of the PC structure in order to further construct a dense WDM (DWDM) system [15]. Zhang and Qiu [16] designed a compact in-plane
n Corresponding author at: Laboratory of Microanalysis, Microscopy and Molecular Spectroscopy, Faculty of Science, Djillali Liabes, University of Sidi Bel Abbes, Algeria. Tel.: +213 6 62 67 29 63. E-mail address: [email protected] (M. Youcef Mahmoud).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.05.032
channel drop filter using a single cavity that supports two high Q modes. By modifying the upper boundary of the drop waveguide, they achieved a high forward dropping. In principle, nearly 100% drop efficiency can be realized and then light can be collected in the normal direction of the PC slab. However, it is a real challenge to integrate this channel drop filter into a planar optical integrated circuit [17]. Thus, significant progress has been made on CDF based devices in the areas of compactness, high spectral selectivity, wide spectral tunability, fast switching, and low-power switching [18,19]. Photonic crystal ring resonators (PCRRs) are also used in the design of CDFs [20]. It can offer scalability in size and flexibility in mode design. Recently, several types of CDF based on 2D PCRR have been proposed using quasi-square PCRR [20], square PCRR [20], dual square PCRR [20], dual curved PCRR [21], hexagonal PCRR [22], 451 PCRR [23], circular PCRR [24], and X-shaped PCRR [25]. In the present paper, we propose a new design of CDF based on 2D PCRRs. The performance of the filter is investigated by using the two-dimensional (2D) finite-difference time-domain (FDTD) technique in triangular lattice photonic crystal (PC) silicon rods. The proposed device has appropriate performance and is very suitable for CWDM multi-channel systems.
2. Design and numerical results In this paper, the structure consists of a two-dimensional (2D) triangular lattice PC composed of silicon (Si) rods embedded in air
180
M.Y. Mahmoud et al. / Optics Communications 306 (2013) 179–184
Fig. 1. (a) W1 line defect PC waveguide, (b) 2D super-cell with a defect (a is the lattice constant), (c) Dispersion curves for TM polarization and the corresponding waveguide mode shown as a pink line in the photonic bandgap region. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
substrate. The refractive index of the rods is 3.46 and the radius of the rods is 0.2a, where a is the lattice constant. As shown in Fig. 1 (a), the W1 line defect waveguide in the PCs is realized by removing a row of dielectric rods along the ΓK direction in a triangular lattice PC that have a fundamental band gap for TM mode (Electric field is perpendicular to x–y plane). Fig. 1(c) shows the dispersion curves of the W1 line defect PC waveguide for the TM polarization through the plane wave expansion (PWE) [26] method using the 2D super-cell approach highlighted in Fig. 1(b).
The waveguide supports a single-mode frequency (normalized) ranging from 0.337 a/λ to 0.442 a/λ below the light-line, where λ is the wavelength of light in free space. To be used in optical communication systems (1550 nm), the lattice constant, a, i.e. the distance between the two adjacent rods, is set as 635.3 nm. Thus the W1 PC waveguide is broadband, with guided singlemode spans from 1437 to 1885 nm. The newly designed channel drop filter based on twodimensional (2D) triangular lattice PC consists of two waveguides
M.Y. Mahmoud et al. / Optics Communications 306 (2013) 179–184
181
Fig. 3. Normalized transmission spectra of the CDF at ports B, C and D respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Schematic diagram of the proposed CDF based on 2D PCRR.
and a single X-shaped PCRR sandwiched between them. As shown in Fig. 2, the top waveguide is called as bus waveguide whereas the bottom waveguide which is formed by removing a row of dielectric rods in ΓK direction but rotated 601 and 1201 from the horizontal is known as drop waveguide. The input, the forward transmission, the forward dropping and the backward dropping ports are labeled as A, B, C and D respectively. To create resonant states of different symmetry [12] and ensure complete power transfer from the input to drop port through the ring resonator, four gray rods (small rods) are reduced to 0.77r and shifted in the ΓK direction of 0.18a. The white rods have exactly the same diameters as all other dielectric rods in PC structure and they are also shifted of 0.248a in the opposite side of the ΓK direction. The transmission characteristics of the proposed CDF are calculated using the two-dimensional (2D) finite-difference timedomain (FDTD) technique [27], with perfectly matched layers (PMLs) absorbing boundary conditions. A TM Gaussian optical source is launched at the input port A and excites both the even and odd states, each with a Lorentzian line shape. Power monitors were placed at each of the other three ports (B, C, D) to collect the transmitted spectral power density after Fouriertransformation. All of the transmitted spectral power densities were normalized to the incident light spectral power density from input port A. The normalized transmission spectra at ports B, C and D are displayed in Fig. 3 as black, red and blue lines respectively. As shown, each of the transmission and the backward dropping efficiencies is absent at the resonant frequency, whereas high forward dropping efficiency is observed. On the other hand, all the power in the bus waveguide is extracted by using resonant tunneling process and well transferred into port C through the resonant ring. In this case, the proposed filter can be regarded as two coupled PCRRs with both forward and backward dropping capabilities depending on the mode parities. As shown from the movie in Fig. 4, the coupled mode in the bottom PCRR ring cavity
rotates in the clockwise direction with the propagating waveguide mode, whereas, the propagating wave in the top PCRR ring cavity propagates counter-clockwise which leads to the forward dropping. Note that the system can perform backward dropping if the modes propagate clockwise in both PCRRs. It is interesting that the forward dropping at the port C predominates, which is obviously different with the dominance of backward dropping at resonance for the traditional resonant single micro-ring structure. 100% drop efficiency and a quality factor larger than 1000 can be achieved while the operating wavelength is 1550 nm. The off-resonance transmission is close to 100% over the entire spectrum, except at the resonant frequency, where it drops to 0%. By comparing our new CDF with the related works in the literature, the spectral selectivity and the quality factor of the proposed one is higher, as shown in Table 1. The electric field patterns of the even mode (symmetric with respect to the plane perpendicular to the W1 line defect waveguide) and the odd mode (asymmetric with respect to the plane perpendicular to the W1 line defect waveguide) are obtained by launching a continuous-wave (CW) source into input port A at the resonant wavelength (λ ¼1550 nm) as shown in Fig. 4(a) and (b) respectively.
3. Demultiplexer design for CWDM systems According to the results shown above, we can design a multiCDF to simultaneously add and drop various wavelength channels from a single multi wavelength source using the resonant tunneling process. To illustrate this concept, three parallel cascaded ring resonators are horizontally coupled to a W1 line defect waveguide. The device consists of one bus waveguide and three drop waveguides with various refractive indexes. The variation of the refractive index has been selected to obtain a shift of the resonant frequency. As shown in Fig. 5, the diameter of small and whole dielectric rods in the PC structure remains constant, whereas the refractive index of region 1, 2 and 3 are 3.61, 3.46 and 3.31 respectively. The incident port and exit ports are labeled
182
M.Y. Mahmoud et al. / Optics Communications 306 (2013) 179–184
Fig. 4. Electric field patterns of the CDF at the resonant wavelength (λ¼ 1550 nm) (a) even mode and (b) odd mode (movie 599 Ko).
Table 1 Comparaison to other devices in the literature. Reference
Drop efficiency (%)
Quality factor
PCRR type
Our work Robinson and Nakkeeran [28] Ma and Ogusu [29] Bai et al. [23] Hsiao and Lee [30] Andalib and Granpayeh [21] Djavid et al. [31] Qiang et al. [20]
100 100 95 90 55 68 99 498
41000 114.6 775 840 423 153.6 52.7 160–1000
X-shaped Circular Diamand 451 Hexagonal Dual curved Quasi-square Quasi-square
Fig. 5. Schematic diagram of the multi-CDF.
as A, B, C, D and E respectively. Broadband light is inserted into the bus and resonant wavelengths specific to each ring resonator are individually demultiplexed into separate drop waveguides. By
reversing the input and output roles, the proposed design can also act as a multiplexer to combine various optical wavelength signals into a single bus waveguide.
M.Y. Mahmoud et al. / Optics Communications 306 (2013) 179–184
183
Fig. 6. Normalized transmission spectra of the multi-CDF at ports B, C, D and E respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The transmission spectra for the proposed structure at ports B, C, D and E are displayed in Fig. 6 as black, red, blue and green lines respectively. We confirmed by numerical calculation that an increase in index causes resonant wavelengths to shift toward longer wavelengths and a decrease in index causes a shift toward shorter wavelengths. On the other words, by increasing (decreasing) the refractive index of the rods, a red shift (blue shift) in resonant wavelength occurs. Resonance properties can be precisely tuned to select specific wavelengths by modulating the lattice constant or by further tuning such as the radius of the rods, the radius of small rods, and/or the refractive index of small rods. The forward dropping efficiencies are 100% at all output channels and the selected wavelengths are 1529.95 nm, 1549.79 nm and 1569.94 nm respectively with spacing of 20 nm corresponding to the ITU-T G.694.2 grid for CWDM systems. The electric field patterns at λ ¼1510 nm, λ ¼1529.95 nm, λ ¼1549.79 nm and λ ¼1569.94 nm are shown in Fig. 7(a), (b), (c) and (d) respectively.
4. Conclusion In this paper, a new channel drop filter based on photonic crystal ring resonators in two-dimensional triangular lattice silicon rods was proposed. 100% forward dropping efficiency and a quality factor of over 1000 can be achieved at maximum transfer efficiency while the operating wavelength is 1550 nm. Based on the proposed component, a multi channel drop filter with 100% drop efficiencies at all output ports can be obtained. The proposed filter provides a possibility of channel drop filter and could be used in coarse wavelength division multiplexing (CWDM) systems.
Acknowledgment The present work was supported by the Ministry of Higher Education and Scientific Research of Algeria.
Fig. 7. Electric field patterns at (a) λ¼ 1510 nm, (b) λ¼ 1529.95 nm, (c) λ¼ 1549.79 nm and (d) λ¼ 1569.94 nm.
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