Optimizing Probabilistic Constellation Shaping for Amplifier-Less Coherent Optical Links

Abstract

Coherent optical systems for short-reach links have seen a boost in their popularity with the standardization of 400ZR. These systems need to be cost and power-efficient, while coping with the ever increase in traffic demand. The use of probabilistic constellation shaping (PCS) can help meet these demands, since it reduces the gap to the channel capacity. However, studies concerning PCS typically include optical amplification; therefore, its gain in amplifier-less systems has been somewhat uncertain. In this work, we focus on an amplifierless coherent optical link employing PCS, reducing both power consumption and system complexity due to the removal of the amplifier. We employ PCS for different modulation orders and bit rates, while optimizing several key parameters such as the FEC coding overhead, modulation depth and clipping. With this comprehensive study, we demonstrate by system simulation and experimental validation that PCS is beneficial even for amplifierless systems, since it introduces residual gains while allowing higher flexibility in defining the net bit rate in a system with fixed baud rate.

Unamplified Coherent Transmission of Net 500 Gbps/Polarization/λ for Intra-Datacenter Interconnects

“…On a single polarization, we transmit 128 (124) Gbaud 16QAM over 2 (10) km under the 6.7% OH HD-FEC BER threshold of 3.8×10 -3 , which represents a net rate of 480 (465) Gbps. Adopting a higher FEC threshold, we demonstrate the transmission of 128 (124) Gbaud 32QAM over 2 (10) km at a BER below the 2.4×10 -2 threshold of the 20% ⁄ )), as defined in [19]. Considering only the IQM non-linear transfer function, the optimum modulation depth is between 0.5 and 0.6, which addresses the tradeoff between modulation loss and non-linear compression [19].…”

“…Adopting a higher FEC threshold, we demonstrate the transmission of 128 (124) Gbaud 32QAM over 2 (10) km at a BER below the 2.4×10 -2 threshold of the 20% ⁄ )), as defined in [19]. Considering only the IQM non-linear transfer function, the optimum modulation depth is between 0.5 and 0.6, which addresses the tradeoff between modulation loss and non-linear compression [19]. However, the employed RF amplifier 1 dB compression point is 2.5 Vpp, which increases the transmitted signal non-linearity and dedicates operating at a lower modulation depth.…”

Silicon Photonic Single-Segment IQ Modulator for Net 1 Tbps/λ Transmission Using All-Electronic Equalization

“…However, 32QAM performs better than 16QAM for the higher line rates due to the bandwidth limitations primarily from the SiP IQM and the RF amplifiers. Although the transmitter pre-emphasis filters precompensate the signal spectrum up to 64 GHz, the strong equalization after 50 GHz diminishes the AWG output signal swing; resulting in worse RF signal quality, smaller driving swing, higher modulation loss, and lower OSNR [17]. The summary of the achieved transmission rates is tabulated in Table I, considering different FEC thresholds.…”

Autoencoder-Optimized Geometric Constellation Shaping for Unamplified Coherent Optical Links

“…This impairment has been identified as a peak-power-constraint (PPC), typical to unamplified systems [4]. In [2], we have demonstrated that the application of digital clipping can effectively reduce the PAPR of complex QAM constellations, thereby highly improving the achievable power budget. An important question then arises: is the typical use of QAM formats the most suitable for unamplified transmission?…”

“…Targeting the reduction of cost and power consumption, the recent coherent short-reach 400ZR standard already accounts for optical unamplified short-reach systems [1]. However, this option severely affects the power budget, while also introducing a new challenging constraint: due to the absence of a transmitter-side optical amplifier (booster), the system becomes highly sensitive to the peak-to-average power ratio (PAPR) [2,3]. This impairment has been identified as a peak-power-constraint (PPC), typical to unamplified systems [4].…”