1356 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 18, SEPTEMBER 15, 2007 Postdetection Adjustable Simultaneous Compensation of DGD and GVD in a 40-Gb/s Optical Single-Sideband System M. A. M. Madureira, D. Fonseca, R. Sousa, M. Violas, R. L. Aguiar, A. V. T. Cartaxo, and P.M. P. Monteiro Abstract—Results on postdetection compensation of group velocity dispersion (GVD) and differential group delay (DGD) in a 40-Gb/s optical single-sideband system are presented. An electrically adjustable transversal filter structure and a microstrip line are used as electrical compensators. Our results show that 408 ps/nm of GVD or 18 ps of DGD, considered separately, are compensated with less than 1.3-dB optical signal-to-noise ratio (OSNR) penalty to back-to-back without compensator, for an extinction ratio (ER) of 6 dB. The simultaneous effect of 12.3 ps of DGD and 374 ps/nm of GVD is compensated with less than 2-dB OSNR penalty, also for ER of 6 dB. Simulations performed support the experimental results. Index Terms—Compensation, optical fiber dispersion, optical single-sideband (OSSB), transversal filters (TFs). I. INTRODUCTION T WO MAJOR optical channel impairments, namely group velocity dispersion (GVD) and polarization-mode disper- sion, become of concern in 40-Gb/s long-haul systems due to the considerable intersymbol interference (ISI) effects they may cause. While GVD can be equalized by static optical means (although temperature dependence [1] and dispersion slope of the equalizer may give rise to a residual uncompensated offset), Manuscript received February 16, 2007; revised June 8, 2007. This work was supported in part by Fundação para a Ciência e Tecnologia (FCT) and in part by Agência de Inovação (AI), in the context of projects “The Most—Transimpedance Highly Efficient Micro&milimetric Wave Optical Smart Transceiver” and “Optronet—Optimized Transponders for Robust Networks.” M. A. M. Madureira is with the Research and Development Department, Nokia Siemens Networks Portugal, S. A., 2720-093 Amadora, Portugal (e-mail: miguel.madureira@siemens.com). D. Fonseca is with the Research and Development Department, Nokia Siemens Networks Portugal, S. A., 2720-093 Amadora, Portugal, and also with the Optical Communications Group, Instituto de Telecomunicações, De- partment of Electrical and Computer Engineering, Instituto Superior Técnico, 1049-001 Lisboa, Portugal. R. Sousa is with Instituto de Telecomunicações—Pólo de Aveiro, Universi- dade de Aveiro, 3810-193 Aveiro, Portugal. M. Violas and R. L. Aguiar are with the Instituto de Telecomunicações—Pólo de Aveiro, Universidade de Aveiro, 3810-193 Aveiro, Portugal, and also with the Departamento de Electrónica e Telecomunicações, Universidade de Aveiro, 3810-193 Aveiro, Portugal. A. V. T. Cartaxo is with the Optical Communications Group, Instituto de Telecomunicações, Department of Electrical and Computer Engineering, Insti- tuto Superior Técnico, 1049-001 Lisboa, Portugal. P. M. P. Monteiro is with the Research and Development Department, Nokia Siemens Networks Portugal, S. A., 2720-093 Amadora, Portugal, with Insti- tuto de Telecomunicações—Pólo de Aveiro, Universidade de Aveiro, 3810-193 Aveiro, Portugal, and also with the Departamento de Electrónica e Telecomuni- cações, Universidade de Aveiro, 3810-193 Aveiro, Portugal. Digital Object Identifier 10.1109/LPT.2007.902996 polarization-mode dispersion is a random effect owing to its de- pendence on the channel and external environmental factors [2] requiring, as a consequence, dynamic compensation. Electrical compensation using microwave waveguides [3] and adjustable electrical dispersion compensator (AEDC) monolithic devices [4] have been proposed with cost and size advantages over op- tical solutions. Additionally, attention has been focused on mod- ulation formats with advantageous spectral characteristics, such as optical single sideband (OSSB), which increase the tolerance to GVD by reducing the spectral occupancy compared to con- ventional optical double-sideband (ODSB) modulation [5]. In a previous letter [6], we discussed the performance of ODSB and OSSB systems considering an AEDC to compensate GVD alone. In this letter, we present results on the simultaneous compensation of GVD and differential group delay (DGD) on a 40-Gb/s OSSB system. Previous results on ODSB [4] report 13 ps of DGD tolerance and 70 ps/nm of GVD tolerance. We achieve 18 ps of DGD tolerance for 2-dB optical signal-to-noise ratio (OSNR) penalty and 272 ps/nm of GVD tolerance with no OSNR penalty. Including a dispersive microstrip transmission line (DMTL) combined with the AEDC, GVD compensation range is further increased. With this arrangement, we achieve the compensation of the simultaneous effect of 12.3 ps of DGD and 374 ps/nm of GVD with less than 2-dB OSNR penalty to back-to-back without AEDC. II. TEST SETUP DESCRIPTION The developed AEDC prototype is described in [6] and [7]. It is based on a five-tap transversal filter (TF) with tap delay of 18 ps. The chip was assembled on a test carrier and all tests and results reported in this letter refer to this prototype. The test setup is shown in detail in Fig. 1. An optoelectrical OSSB filter follows a conventional nonreturn-to-zero ODSB intensity mod- ulation stage [5]. Two different extinction-ratio (ER) values are considered: 10 dB to comply with [8] and a lower value (6 dB) as in our setup the sideband suppression is higher for lower ER, thereby improving system performance [5]. The DGD is intro- duced using an emulator [4], which sets the desired time delay between the two polarizations of the OSSB signal. The polariza- tion controller before the emulator is adjusted for equal power splitting ratio on the two output polarizations—worst-case bit- error-ratio (BER) reading. The GVD is introduced using spools of standard single-mode fiber (SSMF) of various lengths with dispersion parameter of 17 ps/nm/km. The optical power in the SSMF is kept below 0 dBm to minimize nonlinear effects. Vari- able optical attenuators are used to control the OSNR at the re- ceiver input. An optical filter with a bandwidth of 450 pm is used for noise limiting. This bandwidth is enough for the OSSB 1041-1135/$25.00 © 2007 IEEE