The present document relates to an optical communication system. In particular, the present document relates to the alignment of the laser frequency at a transmitter of the optical communication system and the local oscillator frequency at a coherent receiver of the optical communication system. A coherent optical receiver (222) is described. The receiver (222) comprises a reception unit adapted to convert a received optical signal at a carrier frequency into a digital signal using a local oscillator, referred to as LO, having an LO frequency; a spectral analysis unit adapted to estimate a degree of asymmetry of a spectrum of the received optical signal, based on the digital signal; and a transmission unit adapted to transmit an indication of the degree of asymmetry to a near-end transponder (210) comprising a transmitter (211) of the received optical signal.