International Journal of Scientific Engineering and Technology (ISSN : 2277-1581) Volume No.3 Issue No.6, pp : 823-825 1 June 2014 IJSET@2014 Page 823 Optimized Optical Filtering For 40 Gb/s/Channel Optical Differential Quadrature Phase Shift Key (DQPSK), In DWDM Systems For Acces Network S. Benameur 1 , M. Kandouci 1 , C. Aupetit-Berthelemot 2 , 1 Department of electronics University of Djilali Liabes, SIDI BEL-ABBES, ALGERIA 2 Department C2S2. XLIM - UMR CNRS n°7252 Limoges, France Sarah_mail14@yahoo.fr Abstract— The optimization of multiplexing (MUX) and demultiplexing (DMUX) 3-dB filter bandwidths for the optical differential quadrature phase shift key (DQPSK) combined with the return-to-zero (RZ) and carrier-suppressed return- to-zero (CS-RZ) is performed. The study of different optical signals in the 40 Gb/s dense wavelength division multiplexing (DWDM) system, with high spectral efficiency, is shown to be achievable with different filter types for access network. Performances of MUX and DMUX with different filter bandwidth is analyzed to get the best system performance for 40 Gbs/s DWDM transmission system with high spectral efficiency. The limitations imposed by misalignment of filters are also discussed. Keywords: Optical differential quadrature phase shift key (DQPSK), dense wavelength division multiplexing (DWDM), misalignment, optical filter, Bessel, Butterworth. I. Introduction In recent years, several modulation formats have been suggested as alternatives to the commonly used non return-to-zero (NRZ) and return-to-zero (RZ). These include, carrier-suppressed RZ (CS-RZ) [i, ii ], duobinary [iii, iv], and optical differential quadrature phase shift key (DQPSK) . Due to its narrower spectra DQPSK is expected to provide improvements in terms of tolerance to chromatic dispersion (CD) and should allow increased spectral efficiency in dense wavelength division multiplexing (DWDM) systems. DQPSK modulation format is gradually becoming a research focus [i]. However, the relative performance of this modulation format strongly depends on the type of system (fiber type, amplifier spacing, etc.) [v]. In a WDM system, there is additional signal degradation compared to a single channel system. These impairments come from both the multiplexing/demultiplexing, and non-linear impairments in the fiber, such as cross-phase modulation (XPM) and four-wave mixing (FWM). In this paper, we investigate the penalty from multiplexing and then demultiplexing (MUX-DMUX penalty) [vi]. We assess numerically the influence of narrow optical filtering at the transmitter and at the receiver on the performance of a DWDM system for access network. Five channels are considered with a bit rate of 40 Gb/s and a channel spacing of 50 GHz using RZ/CS RZ - DQPSK. The optical filters used in the simulations were 3rd order Bessel (AWG transfer function) and Butterworth (TTF transfer function) filters [vii]. II. Simulation description With DQPSK the data is encoded into one out of four different symbols, thus enabling simultaneous transmission of two bits per symbol. The main benefit of DQPSK is the reduced spectral width, which is about half that of a binary format at the same bit rate. DQPSK systems were sufficiently mature to allow demonstrations with good performance at bit rates above 10 Gbit/s [viii, ix]. The 40 Gb/s DQPSK modulation format, was generated using two Mach-Zehnder modulators (MZM) separately in series way. After the precoding, the branches of the in-phase (I) and quadrature-phase (Q) are respectively sent into two MZMs for four levels phase modulation. The code type of optical DQPSK is produced firstly through two MZMs for the phase modulations, then followed by the clock signal for shaping, thus code types of different duty cycle are obtained (RZ /CS RZ DQPSK), which concludes that three MZMs are needed as shown in figure2 [x]. Figure1. RZ/CS RZ DQPSK transmitter setup. The 40 Gb/s DQPSK signals are generated as described in [x], five DWDM channels spaced by 50 GHz are considered. A pseudo-random bit stream (PRBS) of length 2 7 -1 bits are considered to rigorously assess the performance of DWDM system [iii]. To achieve smaller channel spacing, the DQPSK signals are filtered by a narrow optical filter as MUX. The system set-up is shown in Figure 2 .The transmission line consisted in 20km single mode fiber (SMF) (17 ps/nm/km, 0.2 dB/km). Figure2. The system setup for 5 x 40 Gb/s over 20 km of SMF- 28 transmission.