The Underestimated Challenges of Burst Mode WDM Transmission in TWDM-PON [Invited]

In this paper, the underestimated challenges of the wavelength-division-multiplexed burst-mode operation in the upstream path of time-and-wavelength-division-multiplexed passive optical networks are analyzed. Various challenges are disclosed, their influence on the signal quality is discussed and mitigation proposals are described: Intra-channel crosstalk can arise from optical network units induced background amplified-spontaneous emission noise and inter-channel crosstalk can be caused by non-ideal filter suppression between wavelength channels at the optical line termination receiver. Such crosstalk cases can be counteracted by reducing the burst-signal output power as well as the rival output noise power, i.e. by applying power leveling. A fast frequency drift in burst mode operation is inherent to directly modulated lasers, but depends on the specific laser design, the laser output power and the burst length. Mitigation mechanisms are an optimized non-standard laser design or a specific mode of operation, e.g. by increase the frequency drift during the preamble of the burst. The power dynamic range common optical pre-amplifier as part of the upstream signal receivers need to handle can be in range of up to 40 dB, because of the multi-wavelength channel operation and of the optical distribution network power dynamic. This power dynamic faced at the receiver can not only cause challenges for the optical amplifiers, but also for the burst-mode detectors. Additionally, the physical layer operation and maintenance of the upstream path is challenging too. Each optical network unit laser needs to be either wavelength pre-calibrated which adds undesired costs or cross-wavelength synchronization of ranging windows has to be ensured across various optical line terminations. Otherwise rogue optical network unit behavior in wavelength and time domain will deteriorate system performance with every new optical network unit entering the network. Further, the operation of the optical network unit laser require a wavelength fine tuning mechanism, which can depend on the receiver filter shape and the received signal strength indicator implementation.