What else can an AWG do? Gabriella Cincotti Department of Applied Electronics, University Roma Tre, via della Vasca Navale 84, I-00146 Rome Italy *
[email protected]
Abstract: The present paper aims to describe other functionalities for an arrayed waveguide grating (AWG)-based device, showing that this widely used configuration can be designed not only to frequency multiplex/demultiplex wavelength division multiplexing (WDM) signals, but also to perform the discrete Fourier transform (DFT) and the discrete fractional Fourier transform (DFrFT) of a signal, in all-optical orthogonal frequency division multiplexing (OFDM) systems. In addition 1 × N and N × N phased array switches architectures are described, as well as a new configuration to perform polarization diversity demultiplexing. Finally, a general approach, based on an analogy with the finite impulse response (FIR) filter approach, is presented to design optical modulators for any modulation format, using either phase modulators (PM) or electroabsorption modulators (EAM). ©2012 Optical Society of America OCIS codes: (230.7400) Waveguides, slab; (230.7390) Waveguides, planar; (250.7360) Waveguide modulators; (250.6715) Switching; (070.2025) Discrete optical signal processing.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
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Received 1 Oct 2012; revised 5 Nov 2012; accepted 5 Nov 2012; published 29 Nov 2012
10 December 2012 / Vol. 20, No. 26 / OPTICS EXPRESS B288
18. S. Shimotsu, G. Cincotti, and N. Wada, “Demonstration of a 8x12.5 Gbit/s all-optical OFDM system with an arrayed waveguide grating and waveform reshaper,” in European Conference on Optical Communications (ECOC) 2012 Th.1.A.2. 19. G. Cincotti, “Generalized fiber Fourier optics,” Opt. Lett. 36(12), 2321–2323 (2011). 20. G. Cincotti, “Optical OFDM based on the fractional Fourier transform,” in Proc. SPIE Photonic West, 8284–08, 2012. 21. H. Yamazaki, T. Yamada, T. Goh, and S. Mino, “Multilevel optical modulator with PLC and LiNbO3 hybrid integrated circuit,” in Optical Fiber Communication Conference and Exposition (OFC) 2011. 22. C. Doerr, P. Winzer, L. Zhang, L. Buhl, and N. Sauer, “Monolithic InP 16-QAM modulator,” in Optical Fiber Communication Conference and Exposition (OFC) 2008 PDP20. 23. C. Doerr and C. Dragone, “Proposed optical cross connect using a planar arrangement of beam steerers,” Photon Technol. Lett. 11(2), 197–199 (1999). 24. T. Tanemura, M. Takenaka, A. Al Amin, K. Takeda, T. Shioda, M. Sugiyama, and Y. Nakano, “InP–InGaAsP integrated 1×5 optical switch using arrayed phase shifters,” Photon Technol. Lett. 20(12), 1063–1065 (2008). 25. C. R. Doerr, G. Raybon, Liming Zhang, L. L. Buhl, A. L. Adamiecki, J. H. Sinsky, and N. J. Sauer, “Low-chirp 85-Gb/s duobinary modulator in InP using electroabsorption modulators,” Photon. Technol. Lett. 21(17), 1199– 1201 (2009). 26. G. Cincotti, “Polarization gratings: design and applications,” J. Quantum Electron. 39(12), 1645–1652 (2003).
1. Introduction Flexible optical transceivers are becoming a hot topic in optical communication research, as they are viewed as a possible solution to upgrade current system towards Tb/s capacities, while optimizing the use of physical resources. Many research and development efforts have been directed toward the design and fabrication of multi-carrier transceivers to generate and process optical ultra-high capacity channels (superchannels) that can be adapted for different wavelengths and modulation order, according to the traffic demand and required system performance [1]. OFDM and Nyquist WDM are seen as the most suitable technologies to introduce bandwidth allocation flexibility into core, metro and access networks, by using orthogonal sub-carriers that have rectangular shapes in time or frequency domains, respectively [2]. However, in the most part of actual implementations, signal processing is performed in the electronic domain, using expensive and power consuming analog-to-digital converters (ADC). The present paper demonstrates that is possible to implement signal processing directly in the optical domain, and describes new AWG configurations, that integrate different functionalities for multiplexing/demultiplexing OFDM sub-channels, vector modulating and switching optical signals, onto a single optical planar lightwave circuit (PLC), significantly improving the size and cost, with respect to more traditional optical assemblies. A novel OFDM technique is described, based on fractional Fourier sub-carriers that are orthogonal over a symbol duration, if the corresponding parameters are adequately chosen; in addition, their chirped feature can compensate chromatic dispersion. In time-frequency reference frame, a signal s(t) is represented along the time axis, and the corresponding Fourier transform (FT)
S ( f ) = F {s}( f ) =
∞
s ( t )e
− j 2π tf
dt
(1)
−∞
along the frequency axis; therefore, the Fourier transform operator F can be viewed as a change in the representation of a signal corresponding to a π/2 counterclockwise axis rotation. The fractional Fourier transform (FrFT) is a generalization of the FT, firstly introduced by V. Namias in 1980 [3]
#177226 - $15.00 USD
(C) 2012 OSA
Received 1 Oct 2012; revised 5 Nov 2012; accepted 5 Nov 2012; published 29 Nov 2012
10 December 2012 / Vol. 20, No. 26 / OPTICS EXPRESS B289
Fig. 1. Time-frequency plane coordinates (FT) rotated of an angle pπ/2 (FrFT).
S p ( u ) = F p {s}( u ) = 1 −
j
∞
π
tan p −∞ 2
s ( t )e
π π jπ u 2 + t 2 cot p − 2 ut csc p 2 2
(
)
dt ,
(2)
and it can be interpreted as a projection of a signal s(t) on an axis that forms an angle pπ/2, with 0
