Optical amplification-free deep reservoir computing-assisted high-baudrate short-reach communication.

An optical amplification-free deep reservoir computing (RC)-assisted high-baudrate intensity modulation direct detection (IM/DD) system is experimentally demonstrated using a 100G externally modulated laser operated in C-band. We transmit 112 Gbaud 4-level pulse amplitude modulation (PAM4) and 100 Gbaud 6-level PAM (PAM6) signals over a 200-m single-mode fiber (SMF) link without any optical amplification. The decision feedback equalizer (DFE), shallow RC, and deep RC are adopted in the IM/DD system to mitigate impairment and improve transmission performance. Both PAM transmissions over a 200-m SMF with bit error rate (BER) performance below 6.25% overhead hard-decision forward error correction (HD-FEC) threshold are achieved. In addition, the BER of the PAM4 signal is below the KP4-FEC limit after 200-m SMF transmission enabled by the RC schemes. Thanks to the use of a multiple-layer structure, the number of weights in deep RC has been reduced by approximately 50% compared with the shallow RC, whereas the performance is comparable. We believe that the optical amplification-free deep RC-assisted high-baudrate link has a promising application in intra-data center communications.

12.1 terabit/second data center interconnects using O-band coherent transmission with QD-MLL frequency combs.

Most current Data Center Interconnects (DCI) use intensity modulation direct detection (IMDD) configurations due to their low complexity and cost. However, significant scaling challenges allow coherent solutions to become contenders in these short reach applications. We present an O-band coherent optical fiber transmission system based on Quantum Dot-Mode Locked Lasers (QD-MLLs) using two independent free-running comb lasers, one each for the carrier and the Local Oscillator (LO). Using a comb-to-comb configuration, we demonstrate a 10 km single mode fiber, O-band, coherent, heterodyne, 12.1 Tbps system operating at 0.47 Tbps/λ using 26 λs. We used fewer comb lines (26 λs), faster symbol rate (56 GBaud) and higher constellation cardinality (32 QAM) relative to the highest capacity C-band systems reported to date. Through design, analysis, and experimentation, we quantify the optimum comb line spacing for this use case. We compare potential configurations for increasing data center interconnect capacities whilst reducing power consumption, complexity, and cost.

Applying implicit training to polarization-division-multiplexed coherent optical systems.

We report the use of implicit training-aided channel estimation (ITA-CE) for polarization-division-multiplexed (PDM) single-carrier coherent optical systems. With the concept of ITA-CE, where the training sequence (TS) is arithmetically added onto the information symbols rather than appearing in separate time slots, overhead-free transmission can be achieved with minor optical signal to noise ratio (OSNR) penalty. ITA-CE system enables simple digital pilot-tone based frequency-offset compensation, and inherits the benefits from typical training-aided channel acquisition and data recovery. Furthermore, we address the issue of self-interference to channel estimation (CE) of ITA-CE system in this paper. The data-induced interference can be eliminated by removing the cyclic mean of the transmitted signal, which can accelerate the convergence of the CE and allows smaller power to be allocated to the TS, leading to enhanced system performance. Numerical simulations and experiments for both PDM-QPSK and PDM-16-QAM systems have been conducted to verify the proposed scheme. Comparable performance to the conventional blind equalization system can be achieved when employing the transmitter side pre-distortion approach to the ITA-CE system.