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.