Performance of Kramers–Kronig Receivers in the Presence of Local Oscillator Relative Intensity Noise

There is increasing interest in low-complexity coherent optical transceivers for the use in short-reach fiber links. Amongst the simplest configurations is the heterodyne coherent receiver, using a 3-dB coupler to combine the signal with the local oscillator (LO) laser output, and a single photodiode for detection of each polarization. In this paper, through numerical simulations, we investigate the impact of signal–signal beating interference (SSBI) and LO relative intensity noise (RIN) on the performance of such coherent transceivers. We assess the performance of two methods to mitigate the SSBI: first, the use of high LO laser power, and second, the application of digital signal processing-based receiver linearization, specifically, the Kramers–Kronig (KK) scheme. The results indicate that, in the case of a RIN-free LO laser, a strong LO is effective in mitigating SSBI and achieves a similar performance to that of the KK algorithm. However, the required increase in LO-to-signal power ratio (LOSPR) is significant. For example, a 20 dB higher optimum LOSPR was observed in the 28 Gbaud dual polarization 16 QAM system at an optical signal-to-noise power ratio of 22 dB. The drawback of using such a high LOSPR is the increased penalty due to RIN-LO beating terms, which we next investigated. The lower optimum LOSPR and, consequently, the lower impact of LO RIN on the performance of the KK receiver lead to a reduction in the pre-FEC BER by over an order of magnitude for LO RIN levels above −140 dBc/Hz