Influence of polarization state, baud rate and PMD on non-linear impairments in WDM systems with mixed PM (D)QPSK and OOK
channels
Mohsan Niaz Chughtai,
1,*Marco Forzati,
1Jonas Mårtensson,
1and Danish Rafique
21
Acreo Netlab, Electrum 236, SE-16440 Kista, Sweden
2
Photonic Systems Group, Tyndall National Institute and Department of EE /Physics, UCC, Cork, Ireland
*mohsan.niaz@acreo.se
Abstract: In this paper we numerically investigate nonlinear impairments in a WDM system with mixed PM (D)QPSK and OOK channels. First we analyze the dependence of XPM and XPolM on SOP and baud rate in absence of PMD. In this case we find that the nonlinear impairments are highly dependent on relative SOP between the PM (D)QPSK and neighbouring OOK channels. The dependence on relative SOP is more pronounced in differential detection than in coherent detection. However, with increasing values of PMD this dependence decreases, and non-linear tolerance improves.
2012 Optical Society of America
OCIS codes: (190.3270) Kerr effect; (060.2330) Fiber optic communications.
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1. Introduction
In order to cope with the increase in demand of high capacity fiber optic transmission links, up gradation of 10 Gbps links with 40/100 Gbps channels has driven the industry to deploy polarization multiplexed quadrature phase-shift keying systems with differential and coherent detection (PM (D)QPSK) as alternatives. However the performance of these PM (D)QPSK channels in pre-existing non-return-to-zero on-off keying (NRZ OOK) based dense wave division multiplexed (DWDM) systems is impacted by nonlinearities induced by cross phase modulation (XPM) and cross polarization modulation (XPolM) [1, 2]. The impact of both XPM and XPolM depends on the baud rate of the PM (D)QPSK channel and relative state of polarization (SOP) to the neighbouring NRZ OOK channels [3, 6]. Here we investigate the dependence of XPM and XPolM on relative SOPs for both coherent and differential detection at three different baud rates of 10 Gbaud, 28 Gbaud and 56 Gbaud. Furthermore, we perform an in depth analysis of the nonlinearities in terms of Stokes vectors of the PM (D)QPSK channel after transmission. We find that the system impact of XPM and XPolM is different for differential and coherent detection. We also observe that polarization mode dispersion (PMD) reduces the dependence of nonlinear impairments on relative SOP of the neighbouring NRZ OOK channels.
2. Simulation setup
The setup for simulations using VPI Transmission Maker™ is shown in Fig. 1. Two nested Mach-Zehnder modulators (MZMs) in the polarization multiplexed transmitter were driven by De-Bruijn bit sequences (DBBS) of order 13 and 12 in x and y polarization. Laser sources with 1 MHz linewidth at a wavelength of 1550 nm were used both in the transmitter and as local oscillator. The signal was pulse carved at 50% duty cycle by an extra MZM for generation of return-to-zero signals. The optical fiber link consisted of eight spans of 80 km standard single mode fiber (SSMF) with full inline dispersion compensation. Each span was amplified by ideal (noise free) amplifiers. The attenuation in SSMF was 0.2 dB/km, the dispersion was 17 ps/nm-km and the non-linear coefficient was 1.31 W
-1km
-1. For dispersion compensating fibers (DCFs) the attenuation coefficient was 0.5 dB/km, the dispersion was -85 ps/nm-km and the non-linear coefficient was 3.5 W
-1km
-1. Four NRZ OOK channels were driven by different (PRBS) sequences at 10 Gb/s. The separation between the central PM (D)QPSK channel and the NRZ OOK channels was 100 GHz. The NRZ OOK channels were separated by 50 GHz from each other.
PM RZ DQPSK / QPSK Transmsitter 10 Gb/s NRZ OOK Source 10 Gb/s NRZ OOK Source
10 Gb/s NRZ OOK Source 10 Gb/s NRZ OOK Source
PM RZ DQPSK / QPSK Receiver SSMF
80 km
DCF
70 Ghz 3 dB demux filter
MUX x 8
OSA