IOP PUBLISHING
JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS
J. Opt. A: Pure Appl. Opt. 10 (2008) 075307 (4pp)
doi:10.1088/1464-4258/10/7/075307
Design of an arrayed waveguide grating optical demultiplexer for CWDM applications Khaled Hassan1 , Diaa Khalil1 , Maurine Malak1 and Hanan Anis2 1 Department of Electronics and Communication Engineering, Faculty of Engineering, Ain Shams University, 1 Elsarayat Street, Abbasia, Cairo, Egypt 2 School of Information Technology and Engineering (SITE), University of Ottawa, 800 King Edward, PO Box 450, Stn A, Ottawa, ON, K1N 6N5, Canada
E-mail:
[email protected]
Received 24 February 2008, accepted for publication 19 May 2008 Published 20 June 2008 Online at stacks.iop.org/JOptA/10/075307 Abstract A proposed arrayed waveguide grating (AWG) demultiplexer (DEMUX) design for coarse wavelength division multiplexing (CWDM) networks is described in which two cascaded AWG are used to obtain a flat-top passband with steep roll-off and wide channel bandwidth. Scalar electromagnetic simulations are used to verify the operation of four-and eight-channel devices satisfying the standard International Telecommunication Union (ITU) grid for CWDM. The devices show good channel uniformity with very low crosstalk in the range of −60 dB in the case of the four-channel demultiplexer. Keywords: arrayed waveguide grating AWG, CWDM, integrated optics, optical network
multiplexing/demultiplexing function. A great deal of effort has been done in this direction in the last few years [2–9]. A comparison between this design, other reported PLC designs and TFFs is shown in table 1. One of the strongest candidates in PLCs is the arrayed waveguide grating (AWG) that has already demonstrated its value in DWDM applications. However, AWG is more compatible with DWDM narrow channel separations (1 nm or lower), and very few designs have been proposed for CWDM applications [4, 5]. Actually, a conventional AWG is characterized by a Gaussian-like response in its passband. This response is suitable for DWDM systems where the channel spacing is very small, of the order of 1 nm, and the source wavelength is so stable that the signal only sees the peak of the response. In CWDM, the channel spacing is 20 nm and the passband, according to the ITU G.694.2, is 13 nm to accommodate the wavelength drift of an uncooled laser source. In this work, we propose a new design for an AWG suitable for CWDM applications. The design is based on the use of cascaded AWGs with different periodicities such that the first stage results in a frequency slicing of the whole spectrum of interest while the second stage re-groups the slices to form the required bandwidth for each channel. This technique is
1. Introduction Wavelength division multiplexing (WDM) optical networks are currently implemented in most wide area networks (WAN) and metropolitan area networks (MAN) and their implementation in access networks is growing rapidly. This is mainly motivated by the rapid increase in the bandwidth required by the subscriber, especially with the massive use of the internet and online computing. While dense WDM (DWDM) is adopted for the WAN and MAN networks, access networks are more sensitive to cost and thus the use of the coarse WDM (CWDM) technique could be more suitable for such applications due to the large channel separations which eliminates the need for high cost, well-stabilized laser sources required in DWDM systems. The design of (DE)MUXs compatible with the CWDM standard is a challenging task due to the tight restrictions on the required response. Thin film filters (TFF) are currently used for this application. However, they should be used either with splitters, which increases the system insertion loss, or in a cascade configuration, which degrades the DEMUX uniformity [1]. A more suitable solution, especially for large numbers of channels, would be the use of a planar light wave circuit (PLC) to achieve the 1464-4258/08/075307+04$30.00
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© 2008 IOP Publishing Ltd Printed in the UK
J. Opt. A: Pure Appl. Opt. 10 (2008) 075307
K Hassan et al
Figure 1. Cyclic property of AWG.
Table 1. Comparison between different CWDM filter designs. Parameter
Proposed design
Other PLCs
TFF
Size
Relatively small 2.5 cm × 2 cm 2 AWGs
Large 8.7 cm × 1 cm [5]
Non-integrated optics component
16 Mach–Zehnder interferometers [8] Easy Around −35 dB [5, 8]
More than the number of channels required Requires more components [1] Conventionally around −30 dB [1]
Number of elements to achieve a DE(MUX) Coupling with fiber ribbons Crosstalk
Easy Around −60 dB
similar to that proposed by Dragone [10] for the design of a DWDM AWG filter with steep response. In CWDM, the design is more challenging as both the splitter and combiners of the AWG should have a wide bandwidth to cover the N channel spectrum. With this new design, we demonstrate fourand eight-channel DEMUX designs with crosstalk lower than −60 dB, satisfying the 20 nm channel separation and the 13 nm channel bandwidth required for CWDM systems. This paper is organized as follows. Section 2 discusses the general principles of AWG. Some of its important properties that represent the core of the proposed design are highlighted. Section 3 discusses the design concept of the CWDM demultiplexer. Section 4 contains the simulation results of the proposed design for four-and eight-channel devices. Finally, in section 5, a conclusion is drawn.
3. Design concept Our design is based on the use of two stages of the conventional AWG structure. The first stage, called the primary AWG, is a 1 × N device whose central wavelength is the central wavelength of the middle CWDM channel with a free spectral range (FSR) of 20 nm. The function of this stage is to slice each CWDM channel into N sub-channels. Thus, the response of the first stage resembles that of a DWDM demultiplexer. The second stage, called the secondary AWG, is an N × M router, where M is the number of CWDM channels being demultiplexed. The secondary AWG will direct the N input sub-channels constituting a certain CWDM channel into a different port according to its central wavelength. Due to the cyclic property of the AWG, each output port will have N different sub-channels combined in this port. The combinations of these sub-channels or slices of the spectrum result in an overall wide channel with steep response. The resolution of the channel details can actually increase by increasing the number of sub-channels. This concept is illustrated schematically in figure 2 with the expected response for each output CWDM channel.
2. AWG design The AWG circuit is basically an optical grating structure integrated on a substrate using planar technology. Its main function is to spatially separate different signals with different frequencies. For this reason, it is constructed from an optical power splitter followed by an array of waveguides (arms) and a dispersive combiner. Its dispersion power is due to the wavelength-dependent phase shift that a signal experiences in the AWG arms. The device performance and some of its applications are discussed in detail in [11]. Our design is mainly based on two powerful properties in the AWG, that is: • Periodicity: as the spectral response between the input and any of the output guides has a spectral periodicity or free spectral range (FSR). This FSR represents the minimum wavelength separation between two wavelength channels that map to the same output port. • Cyclic property: in N × M AWG the response of a certain output port depends on the input port in a cyclic way. A special case for an N × N router and in case the separation between output ports is identical to the separation between input ports we get a function similar to a shift register which is illustrated in figure 1.
4. Demultiplexer design and simulation results 4.1. Four-channel demultiplexer To illustrate the basic idea of our design, the slicing concept is applied to demonstrate a four-channel DEMUX with center wavelengths (1550–1570–1590 and 1610 nm) according to the standard ITU G.694.2 CWDM wavelength grid. The primary AWG is thus designed at a central wavelength of 1570 nm with a free spectral range (FSR) of 20 nm. The number of output ports is four, corresponding to four sub-channels with a channel spacing of 4 nm. The primary AWG parameters are summarized in table 2. For the secondary AWG, it is designed for the center wavelength 1580 nm with an FSR of 160 nm. Although the FSR required to accommodate four CWDM channels is 2
J. Opt. A: Pure Appl. Opt. 10 (2008) 075307
K Hassan et al
N sub-channels
Prim. AWG 1xN
dB
N sub-channels
Sec. AWG NxM
M-Channel M λ
Figure 2. Design concept.
0 -10
Transmission (dB)
-20 -30 -40 -50 -60 -70 -80 -90 -100 1.54
1.55
1.56
1.57
1.58 1.59 Wavelength (μm)
1.6
1.61
1.62
Figure 3. Response of four-channel CWDM demultiplexer.
Table 2. Primary and secondary AWG parameters. Parameters
Prim. AWG
Sec. AWG
No. of input ports No. of output ports Central wavelength, λ0 (nm) Channel separation, λ (nm) FSR (nm) Substrate Substrate index, n sub Index contrast, n Waveguides (Buried) Arm length difference, L (μm)
1 4 1570 4 20 SiO2 /Si 1.45 0.007 5 μm × 5 μm 85.29
4 4 1580 20 160 SiO2 /Si 1.45 0.007 5 μm × 5 μm 9.78
Table 3. Connecting grating. Grating 1 Grating order Grating configuration Center wavelength (nm)
76 0, 1, 2, 3 1548
Grating 2
−2 1, 0, 0, 1 1612.727
parameters are chosen such that the device can be fabricated using a low cost ionic-exchange process with potassium salt. The simulation is carried out on an equivalent 2D structure assuming the effective index method (EIM) is used to transform the 3D structure to a 2D one. The device response is shown in figure 3 where the insertion loss is smaller than 4 dB and the worst case ripples level is 1.5 dB. The bandwidth is 13 nm as implied by the standard. The crosstalk is in the range of −60 dB. This crosstalk-enhanced response is due to the cascading of two AWG as noted in [13].
80 nm, we designed it for 160 nm to decrease the insertion loss and enhance the uniformity. The secondary AWG operates in diffraction order indexed by m = 9. Devices operating at such low order use the asymmetric (s-bend) configuration for the array arms as reported in [12]. The four connecting waveguides between the two AWGs are designed as an irregular grating, which is composed of two regular gratings with different grating orders and different configurations. These gratings are described in table 3. A beam propagation method (BPM) simulation is performed to examine the functionality of the previous design. The material parameters used in the simulation are 1.45 for a silica substrate and 0.007 for the index difference. These
4.2. Eight-channel demultiplexer The design for an eight-channel DEMUX is similar to the four-channel one except that we design for a frequency grid rather than a wavelength grid. This is because the FSR changes with wavelength but is constant with frequency, so we designed for equal frequency intervals that, when mapped back to the wavelength, suffer a deviation from the ideal wavelength centers. This is shown in table 4. 3
J. Opt. A: Pure Appl. Opt. 10 (2008) 075307
K Hassan et al
0 -10
Transmission (dB)
-20 -30 -40 -50 -60 -70 -80 -90 -100 1.46
1.48
1.5
1.52 1.54 1.56 Wavelength (μm)
1.58
1.6
1.62
Figure 4. Response of eight-channel CWDM demultiplexer.
These ripples can be reduced by increasing the number of subchannels and also by proper design for the connecting grating between the two AWGs. Good channel uniformity has been obtained in both cases.
Table 4. New wavelength grid. Central wavelength = 1530 (nm) Central frequency = 196.0784 (THz) Ideal wavelength (nm) Deviated wavelength (nm) Ideal wavelength (nm) Deviated wavelength (nm) Ideal wavelength (nm) Deviated wavelength (nm)
1450 1454 1510 1510.3 1570 1571.1
1470 1472.3 1530 1530 1590 1592.4
1490 1491 1550 1550.3 1610 1614.4
References [1] Sasaki H and Okabe Y 2003 CWDM multi/demultiplexer consisting of stacked dielectric interference filters and off-axis diffractive lenses IEEE Photon. Technol. Lett. 15 551 [2] McMullin J N, DeCorby R G and Haugen C J 2002 Theory and simulation of a concave diffraction grating demultiplexer for coarse WDM systems J. Lightwave Technol. 20 758 [3] Iazikov D, Greiner C M and Mossberg T W 2006 Integrated holographic filters for flat-passband optical multiplexers J. Opt. Express 14 3497–3502 [4] Kamei S, Doi Y, Hida Y, Inoue Y and Suznki S 2003 Low-loss and flat/wide-passband CWDM demultiplexer using silica-based AWG with multi-mode output waveguides J. Opt. Soc. Am. [5] Doerr C R, Pafchek R and Stulz L W 2003 Integrated band demultiplexer using waveguide grating routers IEEE Photon. Technol. Lett. 15 1088 [6] Ohki S, Kawajiri Y, Fukuda M, Tachikawa Y and Satoh M 2005 Pass band broadening in molded-glass echelon-grating-based CWDM filter module Regular Papers vol 3 [7] Doerr C R, Cappuzzo M, Gomez L, Chen E, Wong-Foy A and Laskowski E 2004 Planar lightwave circuit eight-channel CWDM multiplexer J. Lightwave Technol. 23 62 [8] Low-crosstalk 4-channel coarse WDM filter using silica-based planar-lightwave-circuit Optical Fiber Communication Conf. Exhibit 2002 OFC 2002 pp 75–6 [9] Very wideband AWG multiplexer and demultiplexer for CWDM systems http://www.phlab.ecl.ntt.co.jp [10] Dragone C 1996 Frequency routing device having a wide and substantially flat passband US Patent Specification 5488680 [11] Smit M K and van Dam C 1996 PHASAR-based WDM-devices: principles, design and applications IEEE J. Sel. Top. Quantum Electron. 2 236–50 [12] Adar R, Henry C H, Dragone C, Kistler R C and Milbrodt M A 1993 Broad-band array multiplexers made with silica waveguides on silicon J. Lightwave Technol. 11 [13] Kamei S, Kaneko A, Ishii M, Shibata T, Inoue Y and Hibino Y 2005 Crosstalk reduction in arrayed-waveguide grating multiplexer/demultiplexer using cascade connection J. Lightwave Technol. 23 1929
This shows that the maximum deviation from the center wavelength is 4.4 nm. The primary AWG is designed to have five output ports corresponding to five sub-channels with spacing of 4 nm; thus the design targets a passband of 20 nm, so a deviation of 4 nm will be tolerable as the required 13 nm passband will still be possible. The simulated response is shown in figure 4. The insertion loss is around 5 dB with the ripples level varying from 1.2 to 3 dB. The crosstalk level is still low around −45 dB. The degradation in crosstalk level is due to increasing the number of channels. The relation between the crosstalk and number of channels was introduced in [13]. A similar design approach for a sixteen-channel CWDM demultiplexer is also possible with a performance close to the eight-channel one. However, the sixteen-channel device will not satisfy the CWDM wavelength grid; rather, it will satisfy a constant frequency grid as noted in the previous eight-channel design.
5. Conclusion We presented a new design for an integrated optical CWDM demultiplexer on a silica substrate. The proposed structure uses the cascade of two AWGs to achieve wide flat-top passbands. The structure is simulated using EIM-enhanced BPM. A fourchannel demultiplexer design shows an insertion loss lower than 4 dB, with a ripple of 1.5 dB, and a crosstalk level of −60 dB. An eight-channel design is also demonstrated with 5 dB insertion loss, 3 dB ripple level and a crosstalk of −45 dB. 4
