Optik 124 (2013) 6013–6016 Contents lists available at ScienceDirect Optik jou rn al homepage: www.elsevier.de/ijleo Performance improvement for N × 80-Gb/s WDM transmission link with optimized alternate polarization Richa Bhatia a , Ajay K. Sharma b,∗ , Jyoti Saxena c a Department of Electronics and Communication Engineering, Ambedkar Institute of Advanced Communication Technology and Research, Delhi 110031, India b Department of Computer Science and Engineering, National Institute of Technology, Jalandhar 144011, Punjab, India c Department of Electronics and Communication Engineering, Giani Zail Singh College of Engineering and Technology, Bathinda, Punjab, India a r t i c l e i n f o Article history: Received 29 November 2012 Accepted 20 April 2013 Keywords: Optical communication High bit rate optical modulation Wavelength-Division-Multiplexing (WDM) transmission Alternate polarization a b s t r a c t In this paper, it is shown that at a high bit rate of 80-Gb/s alternate polarization of adjacent bits in a Wavelength Division Multiplexed (WDM) transmission link improves the system performance in terms of improved Q factor and minimum bit error rate (BER). Alternate Polarization Return to Zero (al-PRZ) further suppresses the non-linear effects at higher power levels of 25 dBm per channel and also improves the transmission length to 640 km for a N × 80-Gb/s WDM system and hence results in an improvement of BER to 10 -20 . © 2013 Elsevier GmbH. All rights reserved. 1. Introduction Transmission of WDM systems over long transmission lengths is affected by chromatic dispersion, polarization mode dispersion (PMD) at high bit rate and fibre non-linearity at high power level. These limitations can be overcome by the choice of appropriate modulation format to a greater extent. The commonly used mod- ulation formats are non return-to-zero (NRZ) and return-to-zero (RZ). For higher bit rates specially designed double modulation based NRZ and RZ modulation formats can be used, which can reduce interference and non-linear effects to some extent. Use of differential phase shift keying (DPSK) improves tolerance to opti- cal noise improving optical signal to noise ratio. Then RZ-DPSK at 10-Gb/s outperforms RZ for its higher tolerance to non-linear effects [1]. To further increase the tolerance to non-linearities, the polarization of every other bit is rotated by 90 ◦ , generating alternate polarization RZ-DPSK (APol RZ-DPSK) [1]. Again, the alter- nate polarization of adjacent symbols in 40 Gb/s RZ-differential quadrature phase shift keying (RZ-DPSK) transmission system can improve significantly system performance through suppression of intra-channel non-linear impairments. [5]. Also at 40-Gb/s it has been proved that al-PRZ offers better results for single channel as well as WDM systems [2]. The al-PRZ based transmission system ∗ Corresponding author. Tel.: +91 181 2690301/02; fax: +91 181 2690932. E-mail address: sharmaajayk@nitj.ac.in (A.K. Sharma). offers improvement in transmission quality through reduction of non-linear effects at 40-Gb/s with a transmission length of 320 km. All the above papers have implemented the WDM system at the bit rate of 40-Gb/s or even lower. In this paper, we have extended the work by increasing the bit rate to 80-Gb/s. Numerical inves- tigations of al-PRZ signals and al-PNRZ signals with orthogonal polarization between adjacent channels are made for 80-Gb/s opti- cal transmission system. The characteristics of al-PRZ and al-PNRZ for WDM transmission over standard single mode fibre (SSMF) are compared with NRZ and RZ formats by means of numerical simu- lations. At the receiver, a conventional non-polarization sensitive receiver is considered. 2. Methodology of alternate polarization signal generation at 80-Gb/s The al-PRZ pulses are generated as shown in Fig. 1. A continu- ous wave (CW) laser emits light at a wavelength of around 1.55 m and is fed to the Mach–Zehnder modulator (MZM) as carrier. A pseudo random bit sequence (PRBS) generator creates the data bit sequence which then is encoded by a RZ coder, which is further fed as data input to the MZM. An 80-Gb/s RZ signal is then generated at the output of MZM. The al-PRZ pulses are then realized by passing these RZ signals through a second modulation stage of polarization modulator. The polarization modulator is presented in Fig. 1. The RZ signal is given to the polarization controller (PC) where its state of polarization is adjusted to a linear polarization at an 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.04.109