Abstract
In this paper, we present a numerical, theoretical and experimental study on the mitigation of Polarization Dependent Loss (PDL) with Polarization-Time (PT) codes in long-haul coherent optical fiber transmissions using Orthogonal Frequency Division Multiplexing (OFDM). First, we review the scheme of a polarization-multiplexed (PolMux) optical transmission and the 2 × 2 MIMO model of the optical channel with PDL. Second, we introduce the Space-Time (ST) codes originally designed for wireless Rayleigh fading channels, and evaluate their performance, as PT codes, in mitigating PDL through numerical simulations. The obtained behaviors and coding gains are different from those observed on the wireless channel. In particular, the Silver code performs better than the Golden code and the coding gains offered by PT codes and forward-error-correction (FEC) codes aggregate. We investigate the numerical results through a theoretical analysis based on the computation of an upper bound of the error probability of the optical channel with PDL. The derived upper bound yields a design criterion for optimal PDL-mitigating codes. Furthermore, a transmission experiment of PDL-mitigation in a 1,000 km optical fiber link with inline PDL validates the numerical and theoretical findings. The results are shown in terms of Q-factor distributions. The mean Q-factor is improved with PT coding and the variance is also narrowed.
Highlights
Taking advantage of the numerous degrees of freedom in an optical fiber is a key solution to the exponentially increasing demand of capacity [1] over long-haul and metropolitan optical fiber networks
This is mainly due to coupling effects and differential group delays such as polarization mode dispersion and rotations of the polarization states in polarization division multiplexed (PDM) systems [4], as well as differential losses between the multiplexed channels such as polarization dependent loss (PDL) in PDM systems [5] and mode dependent loss (MDL) in space-division multiplexing (SDM) systems [6]
We will transmit the PT-coded Orthogonal Frequency Division Multiplexing (OFDM) in the recirculating loop of Fig. 6 and replace the in-line PDL element by an attenuator having the same insertion loss and emulate PDL at the transmitter side by inserting an attenuator in one branch of the PolMux transmitter
Summary
Taking advantage of the numerous degrees of freedom in an optical fiber is a key solution to the exponentially increasing demand of capacity [1] over long-haul and metropolitan optical fiber networks. The increase in the spectral efficiency of a multiplexed optical transmission comes at the detriment of a reduction in performance of the transmission scheme This is mainly due to coupling effects and differential group delays such as polarization mode dispersion and rotations of the polarization states in PDM systems [4], as well as differential losses between the multiplexed channels such as polarization dependent loss (PDL) in PDM systems [5] and mode dependent loss (MDL) in SDM systems [6]. These effects can be modeled using a multiinput multi-output (MIMO) formalism [7] which paves the way to the adaptation of mature digital signal processing (DSP) tools, initially designed for wireless MIMO transmissions and the search for new MIMO techniques specific for the optical fiber channel
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