Kramers-Kronig Receiver With Digitally Added Carrier Combined With Digital Resolution Enhancer

Digital phase reconstruction of directly detected minimum phase (MP) optical signals using the Kramers-Kronig (KK) algorithm has recently drawn attention because of its low hardware complexity. Only a single photodiode is required to detect the intensity of the optical signal and KK relations are used to retrieve the phase information. To make sure the signal detected by the photodiode is MP, a carrier tone needs to be inserted. In this work, we digitally add the carrier tone at the transmitter, omitting the need for additional hardware to generate the carrier tone. However, digitally adding the carrier tone increases quantization noise resulting from the digital-to-analog conversion at the transmitter. A digital resolution enhancer (DRE) can reduce effective transmitter quantization noise by moving the quantization noise outside the signal band. We investigate the applicability of DRE to compensate for the increased transmitter quantization noise caused by the digital carrier tone insertion. We experimentally validate this concept by demonstrating 64-ary quadrature amplitude modulation (QAM) transmission at $25 \,{\mathrm GBd}$, showing that DRE increases transmission performance for a 6 bits digital-to-analog converter (DAC) system. Furthermore, employing DRE allows the reduction of the resolution of the DAC system to 4 or 5 bits with a limited performance penalty.