Clock Recovery Challenges in DSP-Based Coherent Single-Mode and Multi-Mode Optical Systems
We present an analysis of clock recovery algorithms in both polarization division multiplexing systems and mode division multiplexing systems. The impact of inter-polarization time skew and polarization mode dispersion in single-mode fibers, as well as the combined impact of mode mixing and mode group delay spread in multi-mode fibers under different coupling regimes are investigated. Results show that although the clock tone vanishing has a known solution for single-mode systems, in multi-mode
... systems even for low group delay spread, strong coupling will cause clock tone extinction, making it harder to implement an effective clock recovery scheme. So far, the research community has mainly focused on equalization schemes for mode mixing and delay spread mitigation        . Strong mode coupling regime has been shown to be preferable in terms of equalizer impulse response length  and nonlinearity tolerance [19, 20] . However, most experimental demonstrations have been in the weak/intermediate coupling regimes  . So far, very little attention has been paid to the impact of mode mixing on clock recovery performance and feasibility. Typically, clock recovery is performed before dynamic multiple-input multiple-output (MIMO) equalization by extracting a clock tone from the signal and then resampling it, so if the clock tone cannot be extracted due to transmission impairments, the remaining part of the DSP chain may fail  . It is therefore essential to investigate the tolerance and performance of clock recovery for systems employing MDM. In this article, we extend our recent work  and present a detailed numerical analysis of clock recovery for long-haul transmission dual-polarization in a SMF and space-division multiplexing using a FMF in weak, intermediate and strong coupling regimes. We show that although for a SMF, the clock tone vanishing can be dealt with a simple polarization rotation  and compensation of the transmitter time-skew between polarizations , the combined effects of mode coupling and inter-modal dispersion in a FMF can significantly degrade the performance of the timing synchronization. In the strong coupling regime, even with low group delay spread, the clock tone completely vanishes, making timing synchronization challenging. This article is structured as follows. In Section 2, we present a review of clock recovery in coherent optical receivers, showing the basic algorithm implementations. In Section 3, we define the propagation model for single-mode and multi-mode optical fibers. In Section 4, we analyze through simulations the performance of clock recovery in single-mode optical fibers considering inter-polarization time skews and polarization mode dispersion. In Section 5, we analyze through simulations the performance of multi-mode optical fibers under different coupling regimes considering modal dispersion. Finally, the article is concluded in Section 6.