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.

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Using end-to-end deep learning, we experimentally demonstrate the optimized design of geometric constellation shaping for coherent unamplified links. A power budget gain of more than 2 dB is demonstrated for 8-ary constellations.