Abstract
We present both theoretically and experimentally a novel blind and fast method for estimating the State of Polarization (SOP) of a single carrier channel modulated in square Dual Polarization (DP) MQAM format for optical coherent receivers. The method can be used on system startup, for quick channel reconfiguration, or for burst mode receivers. It consists of converting the received waveform from Jones to Stokes space and looping over an algorithm until a unitary polarization derotation matrix is estimated. The matrix is then used to initialize the center taps of the subsequent classical decision-directed stochastic gradient algorithm (DD-LMS). We present experimental comparisons of the initial Bit Error Rate (BER) and the speed of convergence of this blind Stokes space polarization recovery (PR) technique against the common Constant Modulus Algorithm (CMA). We demonstrate that this technique works on any square DP-MQAM format by presenting experimental results for DP-4QAM, -16QAM and -64QAM at varying distances and baud rates. We additionally numerically assess the technique for varying differential group delays (DGD) and sampling offsets on 28 Gbaud DP-4QAM format and show fast polarization recovery for instantaneous DGD as high as 90% of symbol duration. We show that the convergence time of this blind PR technique does not depend on the initial SOP as CMA does and allows switching to DD-LMS faster by more than an order of magnitude. For DP-4QAM, it shows a convergence time of 5.9 ns, which is much smaller than the convergence time of recent techniques using modified CMA algorithms for quicker convergence. BER of the first 20 × 10(3) symbols is always smaller by several factors for DP-16QAM and -64QAM but not always for DP-4QAM.
Highlights
Optical coherent receivers are key to higher transmission rates, where information is imprinted in multiple dimensions of the optical waveform
We present in this subsection numerical simulations where the proposed polarization derotation method is assessed over varying instantaneous differential group delays (DGD), assumed constant for the short period required by the algorithm to find the derotation matrix UROT
We presented a novel method for blind estimation of the State of Polarization (SOP) and polarization rotation for single carrier channels and coherent receivers
Summary
Optical coherent receivers are key to higher transmission rates, where information is imprinted in multiple dimensions of the optical waveform. The convergence speed of the CMA is strongly dependent on the SOP of the received waveform ,and varies a lot with the choice of the initial FIR tap values [2,4,5,6] This has become a major problem and is recently being addressed to comply with the fast transition requirements of emerging data rate adaptive optical packet networks [5,6], burst mode coherent receivers and agile optical network architectures where the receiver can be dynamically reconfigured to quickly drop a wavelength and switch to another channel [4]. The process consists of converting the received waveform into the three dimensional Stokes space and looping over an algorithm looking for desired Stokes states on the Poincare sphere until a unitary polarization derotation matrix can be estimated This matrix is used to initialize the center taps of the subsequent decision directed-LMS polarization equalizer.
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