Intercore crosstalk in direct-detection homogeneous multicore fiber systems impaired by laser phase noise

Abstract

Abstract:The impact of the laser phase noise on the photodetected intercore crosstalk and performance of direct-detection orthogonal frequency division multiplexing multicore fiber systems is experimentally investigated. A new solution to overcome the performance fluctuations over time induced by the combined effect of laser phaser noise and intercore crosstalk is proposed. The solution uses adaptive modulation with extended time memory to estimate the bit loading scheme of each subcarrier from the mean and maximum error vector magnitude evaluated over the last ten blocks of transmitted training symbols. During measurements of up to 90 hours, intercore crosstalk power variation induced by fast laser phase noise variations exceeded 20 dB in both time and frequency, and error vector magnitude fluctuations of 4 dB were observed on a sub-second timescale. It is shown that direct-detection orthogonal frequency division multiplexing multicore fiber based systems employing a typical adaptive modulation solution, in which the bit loading scheme is evaluated from a single set of training symbols, suffer from unacceptable outage probabilities and are unable to counteract the fast power variations of intercore crosstalk and phase noise induced impairments. By extending the system memory used to estimate the bit loading scheme employed by the adaptive technique, an outage probability reduction by one order of magnitude is achieved. This reduction is attained by using the mean of the error vector magnitude evaluated over the last ten blocks of training symbols to estimate the bit loading scheme of subcarriers. Further reduction of the outage probability by four orders of magnitude is also demonstrated using a more conservative approach to estimate the bit loading scheme of the subcarriers. However, this conservative approach, based on the maximum error vector magnitude, may lead to additional loss of the average throughput.

Characterization of the stochastic time evolution of short-term average intercore crosstalk in multicore fibers with multiple interfering cores

“…Previous works showed that the impact of the ICXT on the performance is significantly dependent on the modulation format, data-rate, temporal skew between cores, kind of optical receiver, etc., employed in the MCF system [8,26,27]. In [28], a maximum average crosstalk level of -32 dB to maintain the optical signal-to-noise-ratio (OSNR) penalty below 1 dB for systems using 64 quadrature amplitude modulation (QAM) format was shown.…”

“…More recently, it was concluded that MCF-based systems employing carrier free signals and large temporal skews between cores exhibit nearly constant ICXT power over time [26]. Contrarily, direct detection (DD) MCF systems using carrier supported signals are impaired by ICXT power that may fluctuate significantly over time [26,27]. Thus, short-reach access networks, data-centers or other systems, where MCFs with high core count and low-cost DD receivers are preferable to maximize the spatial information density at a reduced cost, may be particularly affected by the dynamic behavior of ICXT over time and require a performance margin to operate adequately [26].…”

Characterization of Crosstalk-Impaired OOK Signals in WC-MCF Systems with High and Low Skew×Bit Rate

“…1 shows the experimental setup employed in the lab to study the impact of the skew-bit rate product on the photodetected OOK signal. To avoid significant degradation induced by laser phase noise [7], an external cavity laser (ECL) with 100 kHz linewidth is employed. The electro-optic conversion is realized by a Mach-Zehnder modulator (MZM) with 10 dB extinction ratio.…”

“…Firstly, the ICXT generated by a continuous wave signal was characterized by a random variation along the longitudinal direction of the fibre [3] and the corresponding ICXT power was modelled by a chi-square distribution [4,5]. Other works have also shown that the shortterm average crosstalk (STAXT), i. e., the ICXT power measured at the output of the interfered core by an optical power meter in a very short time interval (<< 1 second), when an unmodulated signal is launched into the interfering core, changes randomly over time and frequency [6,7]. The power fluctuations of the STAXT induced by modulated signals were also monitored over time [8].…”

Carrier Beating Impairment in Weakly Coupled Multicore Fiber-Based IM/DD Systems

“…For different kinds of WC-MCF based systems, the XT threshold has been studied, such as adaptive direction-detection orthogonal frequencydivision multiplexing (DD-OFDM) signals [19], the on-off key (OOK) signal [20], [21], the 4-level pulse amplitude modulation (PAM-4) signal [22] and the PAM-8 signal [23]. The combined effect of laser phase noise and intercore XT on IM/DD systems has also been investigated [24], [25]. However, the combined effect of inter-core XT and the frequency offset between optical carriers, which we define as the carrier beating impairment (CBI), has not been fully discussed [26].…”