Digital Signal Processing for Optical Coherent Communication Systems

Abstract

I denne afhandling studeres algorithmer til processering af digitale signaler (digital signal processing - DSP), med det formal at kompensere for fysiske begrænsninger i koherente optiske fiberkommunikationssystemer. De fysiske begrænsninger som adresseres i denne afhandling angår den kromatiske dispersion i fiberen, polarisations demultipleksning, o-set i lyskildernes frekvens og fase, samt fasestj. De undersøgte DSP algorithmer regnes som kerneelementer i koherente digitale modtagere til brug i fremtidige optiske kommunikationssystemer, såsom 112-Gb/s "dual polarization (DP) quadrature phase-shift keying (QPSK)" optiske transmissionssystemer. De vigtigste resultater fra denne PhD afhandling kan opdeles i tre omrader. Først præsenteres en eksperimental demonstration af forbedret tolerance over for fasestj med DSP algorithmer som reducerer støjen vha. en "pilot-tone". Dette er så vidt vides den første eksperimentelle demonstration af høj fasestøjstolerance i 40-Gb/s koherente DP-QPSK systemer baseret pa "vertical cavity surface emitting lasers" (VCSELs). Dernæst præsenteres et pionerforsøg hvori der demonstreres høj tolerance over for spektral begrænsning ("spectral narrowing") i 112-Gb/s DP-QPSK koherente optiske kommunikationssystemer, hvilket har stor betydning for den spektrale effektivitet. Resultaterne fra dette forsøg viser at o-line DSP algorithmer kan reducere den bit error rate (BER) penalty som opstår pga. den spektrale begrænsning. For det tredje undersøges bi-direktionel transmission af "carrier-less amplitude- og fase-modulerede" (CAP) signaler. I denne afhandling fokuseres der på den eksperimentelle implementering af DSP baseret kanal estimering i bi-direktionelle optiske transmissionssystemer med CAP signaler.Denne afhandling indeholder også et forslag til rekonfigurerbare, "ultradense wavelength division multiplexed" (U-DWDM) optiske koherente systemer baseret på 10-Gbaud QPSK. Vi demonstrerer et U-DWDM 1.2-Tb/s QPSK koherent system med en spektral effektivitet pa 4.0 bit/s/Hz. I dette eksperiment anvendes såkaldte "digital decision feed back equalizer" (DFE) og "finite impulse response (FIR) equalizer" algorithmer til at reducere interferensen mellem kanalerne. Denne afhandling undersøger også en såkaldt "parallel block-divided overlapped chromatic dispersion DSP compensation" algoritme. Denne algoritme har den væsentlige fordel at den stiller mindre krav til hardwaren i forhold til konventionelle, serielle kromatiske dispersionskompenserings algoritmer.Det konkluderes at de DSP algoritmer som præsenteres i denne afhandling påviseligt kan forbedre koherente digitale modtagere i den næste generation af optiske transmissionssystemer.In this thesis, digital signal processing (DSP) algorithms are studied to compensate for physical layer impairments in optical fiber coherent communication systems. The physical layer impairments investigated in this thesis include optical fiber chromatic dispersion, polarization demultiplexing, light sources frequency and phase offset and phase noise. The studied DSP algorithms are considered as key building blocks in digital coherent receivers for the next generation of optical communication systems such as 112-Gb/s dual polarization (DP) quadrature phase shift keying (QPSK) optical transmission links.Highlight results presented in this PhD thesis include three areas. First, we present an experimental demonstration of enhanced tolerance to phase noise using pilot-tone-aided phase noise mitigation DSP algorithms. To the best of our knowledge, it is the first experimental demonstration of high phase noise tolerance of 40-Gb/s coherent DP-QPSK systems using vertical cavity surface emitting lasers (VCSELs) as transmitter and local oscillator lasers. Second, in order to fulfill the strict constrains of spectral efficiency, this thesis shows the pioneering experimental demonstration of high spectrum narrowing tolerance 112-Gb/s DP-QPSK optical coherent systems using digital adaptive equalizer. The demonstrated results show that off-line DSP algorithms are able to reduce the bit error rate (BER) penalty induced by signal spectrum narrowing. Third, we also investigate bi-directional transmission of carrierless amplitude and phase (CAP) modulation format signal. In this thesis we focus on the experimental demonstration of DSP channel estimation implementations with CAP signal in the bi-directional optical transmission system.Furthermore this thesis proposes recongurable and ultra dense wavelength division multiplex (U-DWDM) optical coherent systems based on 10-Gbaud QPSK. We report U-DWDM 1.2-Tb/s QPSK coherent system achieving spectral efficiency of 4.0-bit/s/Hz. In the experimental demonstration, digital decision feed back equalizer (DFE) algorithms and a finite impulse response (FIR) equalizer algorithms are implemented to reduce the inter channel interference (ICI). This PhD thesis also investigates a parallel block-divided overlapped chromatic dispersion DSP compensation algorithm. The essential benefit of using a parallel chromatic dispersion compensation algorithm is that it demands less hardware requirements than a conventional serial chromatic dispersion compensation algorithm.In conclusion, the digital signal processing algorithms presented in this thesis have shown to improve the performance of digital assisted coherent receivers for the next generation of optical fiber transmission links