Optica 3, 1220–1227 (2016)

Coherent optical communication schemes are attractive because of their potential for efficient, high-capacity data transmission, however, this comes with the trade-off of more complex and costly equipment for signal generation and detection. Now, Antonio Mecozzi and collaborators have proposed a receiver that combines the advantages of coherent schemes with simple direct detection. An information-carrying signal and a frequency-offset continuous-wave local oscillator (LO) found at the left edge of the signal spectrum are launched into the transmission system. Following direct detection of the total field intensity, the complex-valued electric field of the signal is extracted from the measured photocurrent thanks to the Kramers–Kronig relation linking the phase of the total field to its intensity. Use of the Kramers–Kronig relation holds as long as the total transmitted signal is minimum-phase, which in this scheme requires that the amplitude of the LO is larger than that of the signal. The authors simulate the performance of the coherent receiver in a back-to-back configuration, and find that the receiver behaves in a linear manner for minimum-phase signals. If the power of the LO exceeds the signal power by ∼6 dB, then in theory, bit error rates lower than 10−2 can be achieved with an optical signal-to-noise ratio above ∼16 dB for a 24 Gbaud, Gray-coded, 16 quadrature-amplitude-modulated signal.