Abstract
The equalization enhanced phase noise (EEPN), caused by the interaction of the chromatic dispersion (CD) with the phase noise of the local oscillator (LO), has been extensively studied for single-mode optical communication systems. Few-mode fiber (FMF) transmission systems introduce a new channel impairment, the differential mode delay (DMD), which also creates EEPN and hence limits the maximum transmission distance of those systems. In this work, we numerically investigate the optical signal to noise ratio (OSNR) penalties caused by the EEPN in a 3-mode FMF transmission system at 25 GBd for quadrature phase-shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 32-QAM and 64-QAM modulation formats when using the blind phase search (BPS) carrier phase recovery (CPR) algorithm, which has been demonstrated to be both robust and suitable for optical communication systems. Our numerical study assumes a short-span of FMF, modeled in the weakly-coupled regime, and includes two cases; the use of ideal mode-selective de/multiplexers at both ends of the FMF span (model A), and the use of ideal non-mode-selective de/multiplexers (model B). The results show that the EEPN has almost no effect in model A. However, EEPN produces a severe penalty in model B with the onset of the OSNR degradation starting for a DMD spread of the impulse response of about 100 symbols for all modulation formats investigated. The distribution ratio of the amount of phase noise between the transmitter and receiver lasers is also assessed for model B and we confirm that the degradation is mainly due to the phase noise of the LO.
Highlights
Coherent transmission systems using multi-mode fibers (MMFs), and in particular few-mode fibers (FMFs) as a subset thereof, have recently attracted much research attention due to the capacity enhancement enabled by transmitting multiple data streams on a set of orthogonal spatial modes [1,2,3]
Compared to our previous work [24], here we describe in detail the optimization procedure of the blind phase search (BPS) carrier phase recovery (CPR) algorithm and extend the analysis to investigate the impact of the phase noise ratio between the transmitter and local oscillator (LO) lasers for quadrature phase-shift keying (QPSK) and 16-quadrature amplitude modulation (QAM) modulation formats
We have numerically investigated the impact of the enhanced phase noise (EEPN), in terms of the optical signal to noise ratio (OSNR) penalty, in a
Summary
Coherent transmission systems using multi-mode fibers (MMFs), and in particular few-mode fibers (FMFs) as a subset thereof, have recently attracted much research attention due to the capacity enhancement enabled by transmitting multiple data streams on a set of orthogonal spatial modes [1,2,3]. Regarding the EEPN in FMF transmission systems, Shieh [16] analytically derived a simple expression for the signal to noise ratio (SNR) penalty, which grows linearly with the total amount of the DMD and the LO phase noise, for the worst case of total coupling between the two modes. We numerically assess the impact of the EEPN optical signal to noise ratio (OSNR) penalty on a typical 3-mode short-span FMF transmission link with M-quadrature amplitude modulation (QAM) formats at 25 GBd when using a MIMO time-domain equalizer. Non-mode-selective DE/MUXes (model B), even for a short-span of FMF modeled in the weakly-coupled regime, induce a severe penalty due to the EEPN which makes the system phase noise requirements more stringent as the spread of the impulse response increases.
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